ISTANBUL — The opposition Syrian National Council issued a cry for help Sunday to save Homs and other cities from attacks by President Bashar al-Assad's regime, and repeated its call for UN intervention.

"The country is under a violent attack, especially Homs," said Abdel Basset Sayda, the leader of the SNC, Syria's main opposition group.

"This criminal regime is still trying to commit atrocities" in Homs and elsewhere, including the central town of Rastan and the northern city of Aleppo, he told journalists.

The Syrian army tightened its grip Sunday on Homs, assaulting the besieged central city with shelling and gunfire.

SNC spokeswoman Bassma Kodmani said the situation there was growing increasingly desperate.

"The city is left with no food, no electricity, no water, no communications," she told journalists.

Other towns, including Rastan, "are living through tragic hours and days. We are calling for immediate humanitarian intervention in favour of the people of Syria", she added.

Sayda and Kodmani both repeated the SNC's call for the United Nations to pressure Assad's regime using Chapter VII of the UN charter, which allows measures to be imposed on a country under penalty of sanctions or force.

"The next step needs to be a resolution under Chapter VII, which allows for the use of all legitimate means, coercive means, embargo on arms, as well as the use of force to oblige the regime to comply," Kodmani said.

But she added that UN peacekeepers could only be sent into Syria with the regime's blessing, in accordance with the UN's own rules.

The SNC's former leader and current political bureau chief Burhan Ghalioun on Saturday called for the UN to send armed peacekeepers into Syria, after UN observers suspended their mission in the country because of an escalation in fighting and the risk it posed to the unarmed, 300-strong team.