Syria declared Morocco's ambassador persona non grata on Monday, the foreign ministry said, in a tit-for-tat move hours after Rabat expelled Damascus's envoy to the North African country.

"Syria has just considered the accredited Moroccan ambassador to Syria persona non grata," ministry spokesman Jihad Makdissi said via Twitter, after Morocco demanded that Syrian ambassador Nabih Ismail depart the country.

Syria is in a state of all-out civil war and all sides must respect international humanitarian law or risk facing war crimes prosecutions, the Red Cross said on Monday.

The International Committee of the Red Cross had previously regarded only the areas around Idlib, Homs and Hama as zones of non-international armed conflict -- the diplomatic term for civil war.

Russia on Monday accused Western powers of using "blackmail" to get its backing for possible U.N. Security Council sanctions against Syria over the regime's crackdown on an armed opposition.

"To our great regret, we are witnessing elements of blackmail," Foreign Minister Sergei Lavrov told a news conference over moves to end 16 months of violence that the opposition says has claimed more than 17,000 lives.

Fierce fighting engulfed Damascus on Monday, inching closer to the heart of the Syrian capital as troops shelled pockets of resistance in the south, east and west, activists said.

The unprecedented escalation was described as a "turning point" in Syria's 16-month uprising by a Local Coordination Committees (LCC) activist based in the central neighborhood of al-Midan.

A previously unknown group issued on Monday a statement calling on Syrian refugees to leave Lebanon immediately, media reports said.

The group calling itself al-Rida Forces said that all the Syrians currently on Lebanese territories should leave the country.

Kofi Annan and Ban Ki-moon head for Russia and China on Monday to press the two U.N. Security Council doubters to back tougher action against President Bashar Assad to halt the slaughter in Syria.

The visits by the U.N.-Arab League envoy and the U.N. leader come at a crucial new stage in the 16-month old conflict. The Security Council has until Friday to renew the U.N. mission in Syria but is divided over Western calls to add sanctions.

A Russian ship that tried to supply attack helicopters to Syria last month before being forced back was Sunday sighted sailing back home after unexpectedly starting a new voyage.

The privately-chartered Alaed had to return to Russia after its initial attempt to deliver the controversial cargo to President Bashar Assad's regime in June was exposed by the U.S. State Department.

Iran, the main regional ally of Syria, said on Sunday it is ready to host a meeting between Damascus and its opponents aimed at solving the country's conflict.

"Iran is ready to host the Syrian opposition for dialogue with the Syrian government," Foreign Minister Ali Akbar Salehi told Arabic-language Al-Alam television.

Syrian peace mediator Kofi Annan was due in Moscow on Monday for talks with President Vladimir Putin that come amid growing pressure on Russia to finally back the ouster of President Bashar Assad.

The Kremlin said Sunday the U.N.-Arab League envoy would arrive in Moscow on Monday and meet Putin the following day for talks in which "Russia will underscore its support the peace plan of Kofi Annan."

Syria's regime on Sunday denied its forces used tanks and helicopters in an assault on Treimsa, saying what happened in the central village was the result of clashes with rebels and not a "massacre."

Foreign Ministry spokesman Jihad Makdissi said 39 people were killed in Treimsa on Thursday, all but two of them armed men, and that government forces only used light weapons to target five buildings.