Speaking at a press conference in Paris on Tuesday, the UN Secretary General called on both forces in Syria to end the fighting that has destroyed large areas of the country's major cities and claimed tens of thousands of lives.

"It is unbearable for the [Syrian] people to continue like this," Ban told reporters. "That is why I have conveyed to the Syrian government a strong message that they should immediately declare a unilateral ceasefire."

Ban's plea for peace to Syrian President Bashar Assad came just hours after NATO's secretary general told reporters in Brussels that the multinational organization would defend member-nation Turkey in the event of an attack by Syria.

"We have all necessary plans in place to protect and defend Turkey if necessary. We hope it won't be necessary," NATO Secretary General Anders Fogh Rasmussen said.

"We hope that all parties involved will show restraint and avoid an escalation of the crisis. The right way forward in Syria is a political solution," said Rasmussen.

Turkey and Syria have been exchanging artillery and mortar fire across their border since a shell launched from Syria landed on Turkish soil, killing five civilians.

Syrian army, opposition tighten grip on strongholds

While Western leaders planned an end to the Syrian conflict, forces from both sides strengthened their hold on key areas of besieged cities.

Rebels claimed control along a major highway in northern Syria after a two day battle, according to the head of the Britain-based Syrian Observatory for Human Rights.

"Syrian rebels now fully control the key city of Maarat al-Nouman," said Rami Abdel Rahman, referring to a city in the northern province of Idlib.

Meanwhile in the city of Homs, President Assad's army entered an opposition-controlled district, forcing many of the insurgents to flee.

"The catastrophe is that there are 800 families trapped in Homs. It will be an unprecedented massacre if they take over the district," an activist, who identified himself as Abu Bilal, told the news agency AFP.

Around 32,000 people have been killed since the uprising against the Syrian President Bashar Assad first began in March 2011. Over 330,000 Syrian refugees have registered with the United Nations Refugee Agency.

kms/rc (dpa, AFP, Reuters)