India's strikes overnight on what it described as "terrorist infrastructure" in Pakistan and Pakistan-administered Kashmir mark a severe escalation in a decades-old conflict that has flared up after a deadly attack in India-administered Kashmir last month.

That attack, which killed 26 mostly Indian Hindu tourists, was the deadliest involving civilians in India since 2008, and has sparked a major escalation in hostilities between the two South Asian nuclear-armed neighbors.

Pakistani authorities said Wednesday that India's strikes killed at least 26 people, including at least four children, and injured 46 others.

"Attacking our civilians will not go unanswered. They will be held responsible for such a cowardly act," Pakistan's Defence Minister Khawaja Asif told DW.

Asif added that all of the locations India had struck were "civilian," dismissing New Delhi's claims that the targets were terrorist sites as "absolutely baseless and a pack of lies."

Pakistan vows 'full' retaliation

Pakistan's military, meanwhile, said its forces shot down multiple Indian fighter jets.

"Our forces have already responded to the aggression and shot down five of their jets and a combat drone. Don't underestimate Pakistan's response, it will be full, rather more than what they are doing," Asif warned.

In a statement Wednesday after a meeting of its National Security Council, Pakistan said it "reserves the right to respond (...) to avenge the loss of innocent Pakistani lives" in accordance with the United Nations Charter.

"India violated our sovereignty, and we have a right to self defense. We will give India a befitting response as they targeted our women and children," Aqeel Malik, Pakistan's Minister of State for Law and Justice, told DW.

Dr Marco Longobardo, Reader in International Law at the University of Westminster, said India's attack is "unlawful under international law."

He explained that the UN Charter allows states to use armed force in self-defense against an armed attack and that the response "must be proportionate and strictly necessary."

"India allegedly used armed force in response to a terrorist attack that terminated several days ago. Accordingly, the response is not strictly necessary, but rather, it has a punitive goal and, ultimately, it is unlawful," Longobardo told DW.

Strikes trigger fears of spiraling military hostilities

The Indian strikes on targets in Pakistan's most populous province, Punjab, mark the first such attacks since their last full-scale war more than 50 years ago, raising concerns that military hostilities could spiral out of control.

Amit Ranjan, a research fellow at the Institute of South Asian Studies at the National University of Singapore, told DW that he believes "the fight is going to escalate" and that calming the situation depends on how much "the two countries decide to escalate" this episode.

Elizabeth Threlkeld, director of the South Asia program at the Washington-based Stimson Center, agreed that the incident is the most significant between the two nuclear rivals in years.

She pointed to the fact that the attacks struck Punjab, deep inside Pakistani territory, and also hit mosques where "civilian casualties were reportedly sustained."

"The potential for escalation is considerable, particularly in light of initial reports of rapid retaliation from Pakistan and the possible downing of aircrafts,” Threlkeld told DW.

International law expert Longobardo said that although Pakistan is legally entitled to retaliate, its response must be "proportionate and necessary."

"Both India and Pakistan must settle their dispute in a peaceful way, restraining from further uses of armed force," he said.

"The arguments by India and Pakistan resonate the unlawful rhetoric of self-defense as punishment that must be opposed, not only because it is wrong under international law, but also because it leads to escalation," Longobardo underlined.

Calls to stem further escalation

In addition to the strikes overnight, India and Pakistan in recent days have engaged in intense shelling and heavy gunfire along much of their de facto border in the disputed region, according to police and witnesses who spoke to Reuters.

India accuses Pakistan of supporting militant infiltration into India and across the Line of Control, the de facto border that divides the disputed Kashmir region, which both countries claim full sovereignty over. Pakistan denies those allegations.

Pakistani diplomat Maleeha Lodhi, who served as the country's ambassador to the United States, the United Kingdom and the UN, told DW that "the situation now needs to be controlled as further escalation in a nuclear environment is fraught with great risks."

The international community, including the EU, UN and US, has urged both parties to ease the rising tensions.

The US has said that Secretary of State Marco Rubio has spoken to national security advisors from both India and Pakistan to urge them to "maintain open lines of communication and avoid escalation."

US President Donald Trump, meanwhile, has said that he hopes the fighting "ends very quickly," calling the flare up "a shame."

Lodhi pointed out that a US intervention might be required to prevent a "major escalation."

"Past crises between India and Pakistan have always been defused by the US," she said.

Edited by: Karl Sexton