Keeping aside diplomatic niceties, Iran has reacted in the strictest possible manner against Sunni militants' activities along its southeastern province of Sistan-Baluchestan, bordering Pakistan.

Jaish al Adl (the Army of Justice), a Sunni militant group based in Pakistan, killed at least ten Iranian border guards last month. The guards were shot with long-range guns, fired from inside Pakistan.

Jaish al Adl has previously claimed responsibility for attacks that killed Iranian troops in 2013 and 2015. The anti-Iran and anti-Shiite militant group is fighting against what it says is discrimination against Sunni Muslims in Pakistan's southwestern Baluchistan province. Sunni militant groups consider Shiites as apostates.

'Unacceptable'

On Monday, May 8, Iran's army chief said Pakistan must confront Sunni militants or his country would hit their bases itself.

"We cannot accept the continuation of this situation," Major General Mohammad Baqeri, the head of the Iranian armed forces, was quoted as saying by state news agency IRNA.

"We expect the Pakistani officials to control the borders, arrest the terrorists and shut down their bases," Baqeri added.

"If the terrorist attacks continue, we will hit their safe havens and cells, wherever they are," he said.

Last week, Iran's Foreign Minister Mohammad Javad Zarif visited Islamabad and asked Prime Minister Nawaz Sharif to improve border security. Islamabad promised to deploy additional troops along the Pakistani-Iranian frontier.

Pakistan's regional isolation

Iranian-Pakistani relations have been tense for quite some time, but lately there has been a sharp rise in hostility between the two neighboring countries. Experts say that Islamabad's alleged support to Sunni militant groups, most of which operate freely inside Pakistan, is one of the reasons behind the deteriorating ties.

Shia-majority Iran is not the only country in the region that is unhappy with Islamabad's handling of Sunni militants; Afghanistan and India have long accused Pakistan's leadership of backing terrorist organizations which, they claim, are used by the nation's military establishment to create unrest on their soil and gain geopolitical leverage.

There have been severe clashes between Afghan and Pakistani troops along the Afghan-Pakistani border, and the situation is not very different on the so-called Line of Control, the Indian-Pakistani border along the volatile Kashmir region . 

But, for Iran, the main concern regarding Pakistan is Islamabad's renewed defense alliance with its arch-rival Saudi Arabia.

Despite the parliament's decision last year against becoming a party to the intensifying Saudi-Iranian conflict in the Middle East, particularly in Yemen, Islamabad last month approved the appointment of Raheel Sharif, the country's former army chief, as head of the 39-member Saudi-led military coalition. Riyadh says the Muslim nations' alliance was formed to fight terrorism in the region, but experts point out that it is primarily an anti-Iran grouping. Naturally, Tehran is not part of the coalition.

Sattar Khan, DW's correspondent, reports from Islamabad that Prime Minister Nawaz Sharif's government has undertaken a diplomatic initiative to allay Tehran's concerns following Raheel Sharif's controversial appointment. But the efforts are unlikely to yield results, says Khan.

"Saudi Arabia and Iran do not trust each other. Also, Riyadh does not want Islamabad to be neutral in the conflict; it wants its full support. In this scenario, how can Pakistan's diplomatic drive be successful?" Aman Memon, a former professor at the Allama Iqbal Open University in Islamabad, told DW.

Dependence on Saudi Arabia

In 2015, Riyadh formally requested Pakistan to provide combat planes, warships and soldiers to support the Arab coalition in fighting Iran-backed Houthi rebels in Yemen. But the South Asian country's lawmakers voted to remain neutral in the conflict, albeit PM Sharif later clarified in a televised speech that in the case of an aggression against Saudi Arabia, Pakistan would take Riyadh's side.

Saudi Arabia, which is one of Pakistan's biggest financers, was unhappy with Islamabad's reluctance to join the coalition against Yemen's Shiite rebels. The Arab kingdom has been involved in a two-year-long campaign of airstrikes against Houthi rebels, who have taken over swathes of territory in Saudi Arabia's southern neighborhood, raising concerns in Riyadh about a potential Shiite uprising in the kingdom. Saudi Arabia also fears that Iran is working to increase its influence in the region.

Experts say that Pakistan's economic dependence on the Arabian country is also a reason behind its support for Riyadh and Raheel Sharif's appointment. They claim that Pakistan already has troops in Saudi Arabia in an assisting role. But with Sharif taking charge of the alliance, the troops could be directly involved in the battle.

The Arab countries' coalition also has the backing of the United States. In the past few years, Islamabad has drifted away considerably from Washington but analysts say that both countries still have many common strategic interests in the region. Also, the Pakistani military heavily depends on US financial assistance.

"Recently, a US official visited Saudi Arabia and expressed his country's support to the Saudi alliance. He also lampooned Iran. So the objectives of this grouping are pretty clear," Sabir Karbalai, an Islamabad-based analyst, told DW.

The expert, however, added that Islamabad should have remained neutral in the Saudi-Iranian power struggle.

Deteriorating ties

Islamabad's over-enthusiasm to appease Riyadh could further exacerbate its relations with Tehran.

Iranian President Hassan Rouhani visited Pakistan in March last year in an attempt to convince Pakistani authorities to remain neutral in the Middle Eastern conflicts. Iran is aware of the concerns and limitations of its ties with Pakistan, but analysts say it still wants to maintain "normal" relations with Islamabad.

Farhan Hanif Siddiqui, an International Relations expert at the Quaid-i-Azam University, believes that Pakistan needs to assure Iran that the Saudi alliance is not against any country.

"It is vital for Pakistan to convince Iran that the Saudi-led alliance is only against al Qaeda and the so-called "Islamic State." If Tehran continues to believe that the Saudi coalition is targeting the regimes in Iraq and Syria, Pakistan's diplomatic efforts will be in vain," Siddiqui told DW.

Sectarian strife

The South Asian country's intelligentsia and civil society have voiced their displeasure and concern over Raheel Sharif's role in the Saudi alliance and Islamabad's direct involvement in the conflict.

Pakistan's support to Saudi Arabia is likely to increase the Sunni-Shiite tension in the South Asian country. Analysts believe that the Sunni militant groups will feel further emboldened by the fact that the ex-army chief now heads the Saudi-led alliance.

The sectarian strife in Pakistan has been ongoing for some time now, with militant Islamist groups unleashing terror on the minority Shiite groups in many parts of the country. Most of these outfits, including the Taliban, take inspiration from the hard-line Saudi-Wahabi Islamic ideology.

"For Pakistan's Islamic fundamentalists, the country is already a 'Sunni Wall' against Shiite Iran," Siegfried O. Wolf, director of research at the Brussels-based South Asia Democratic Forum (SADF), told DW in an interview.

"The policy of containing the Shiite influence in the region was seriously affected after the collapse of the Sunni Taliban regime in Afghanistan in 2001 and the subsequent overthrow of Saddam Hussein's government in Iraq. These events created a power vacuum which is now being increasingly filled by Tehran. Saudi Arabia does not want to see the rise of Iran and will continue to do anything to ensure Sunni dominance," he underlined.

Additional reporting by Sattar Khan, DW's Islamabad correspondent.