Armoured reinforcements poured into Homs as President Bashar al-Assad's forces bombarded the Syrian city for a fourth day, opposition sources said on Thursday, worsening the humanitarian situation and prompting a new diplomatic push from Turkey.
Turkish foreign minister Ahmet Davutoglu told Reuters before flying to Washington for talks on Syria that Turkey, which once saw Assad as a valuable ally but now wants him out, could no longer stand by and watch.
He said Turkey wanted to host an international meeting to agree ways to end the killing and provide aid.
"It is not enough being an observer," he said. "It is time now to send a strong message to the Syrian people that we are with them," he added, while refusing to be drawn on what kind of action Turkey or its allies would be prepared to consider.
Scores were killed in Homs on Wednesday, according to the opposition, drawing comparison with the plight of the city of Benghazi which triggered Western attacks on Libya last year and accelerating a global diplomatic showdown whose outcome is far from clear.
Activists said that at least 40 tanks and 50 infantry fighting vehicles accompanied by 1,000 soldiers were transported from the nearby border with Lebanon and from the coast and deployed in Homs.
Large Sunni neighbourhoods that have been the target of the heaviest rocket and mortar bombardment by Alawite-led forces loyal to Assad remained without electricity and water and basic supplies were running low, activists in Homs said.
There was no comment from the Syrian authorities, who have placed tight restrictions on access to the country and it was not possible to verify the reports.
"We have seen in the last 24 hours incursions into neighbourhoods such as Khalidiya, Bab Amro and Inshaat. Tanks went in after heavy bombardment and then pulled back," activist Mohammad Hassan told Reuters by satellite phone.
Mazen Adi, a prominent Syrian opposition figure who fled to Paris several weeks ago, said rebels loosely organised under the Free Syrian Army were fighting back and staging hit-and-run guerrilla attacks against loyalist forces in Homs. "The Free Syrian Army is still managing to hit strategic targets in Homs, such as the secret police headquarters," Adi said.
"The regime cannot keep tanks for long inside opposition neighbourhoods because they will be ambushed, and it is retaliating by hysteric bombing that is killing mostly civilians and with mass executions."
He was referring to the reported killing of three unarmed Sunni families in their homes on Wednesday by militiamen loyal to Assad and known as 'shabbiha'.
Adi said that unlike a military onslaught on Hama in 1982 that razed large sections of the city and finished off armed resistance to Assad family rule, Homs was a bigger metropolis and rebels still had lots of cover.
The Syrian opposition intensified calls for international intervention to protect civilians. Activist-in-exile Massoud Akko said Turkey and Western countries needed to organise an airlift to Homs and other stricken cities and towns that have borne the brunt of five months of a sustained military crackdown to put down a mass protest movement against Assad's rule. "What the people of Homs need right now is basic supplies such as medicine and baby food. This could be done by air drops into Homs similar to what the United States did in Iraqi Kurdistan in the 1990s," Akko said.
"It is not enough to say to this regime 'stop the killings', because it won't listen. We are dealing with a system based on political prostitution. The regime is acting as if it is not attacking Homs at all and says the bombardment the whole world is seeing is being done by terrorists."
A statement by the Syrian Revolution General Commission activists' group said friendly countries should call for "an immediate halt to the shelling of cities and residential neighbourhoods", establish safe corridors to supply humanitarian assistance to stricken regions and support the Free Syrian Army.
Syria's position at the heart of the Middle East, allied to Iran and home to a powder-keg religious and ethnic mix, means Assad's opponents have strenuously ruled out the kind of military action they took against Gaddafi.
Russia and China, which let the United Nations support the air campaign in Libya, provoked strong condemnation from the United States, European powers and Arab governments when they vetoed a much less interventionist resolution in the Security Council last week that called on Assad to step down.
Moscow sees Assad as a buyer of arms and host to a Soviet-era naval base. For both Russia and China, Syria is also a test case for efforts to resist UN encroachment on sovereign governments' freedom to deal with rebels as they see fit.
Campaigning for next month's presidential election that he is certain to win, Prime Minister Vladimir Putin, who first won the presidency after storming the rebel Russian city of Grozny, said: "A cult of violence has been coming to the fore in international affairs ... This cannot fail to cause concern.
"We of course condemn all violence regardless of its source, but one cannot act like an elephant in a china shop. "Help them, advise them, limit, for instance, their ability to use weapons but not interfere under any circumstances." It is unclear what Turkey, a NATO member and rising Muslim, democratic force in the Middle East, could do to bring Moscow into any international initiative alongside those regional and world powers which have sided with the rebels against Assad.
Turkish Prime Minister Tayyip Erdogan, who had described the Russian and Chinese veto at the U.N. as a "fiasco", telephoned outgoing Russian President Dmitry Medvedev on Wednesday.
The Kremlin said Medvedev told Erdogan that the search for a solution should continue, including in the Security Council, but that foreign interference was not an option.
Medvedev also spoke with French President Nicolas Sarkozy asking him and other Western countries to avoid "hasty, unilateral moves" towards Syria, the Kremlin said.
Officials in Washington said they hoped to meet soon with international partners to consider how to halt Syria's violence and provide humanitarian aid.