Syrian President Bashar al-Assad pressed on with a tank onslaught against a city on Monday, but was plunged deeper into international isolation by Arab neighbours who denounced his violent crackdown and recalled their envoys from Damascus.

Assad's five-month campaign against opponents has emerged as one of the bloodiest episodes of the wave of unrest sweeping the Arab world this year. Violence has worsened sharply in the past week after Assad ordered tank assaults on two cities.

Other Arab leaders had been cautious about criticising one of their peers, but Saudi Arabia's King Abdullah broke the silence with a rare intervention overnight, demanding an end to the bloodshed and recalling the Saudi ambassador from Damascus.

Hours later Kuwait and Bahrain recalled their envoys too.

Syrian tanks and troops poured into the eastern Sunni city of Deir al-Zor in the latest stage of a campaign to crush centres of protest against 41 years of repressive rule by the Assad family and domination by his Alawite minority community.

"Armoured vehicles are shelling the al-Hawiqa district heavily with their guns. Private hospitals are closed and people are afraid to send the wounded to state facilities because they are infested with secret police," Mohammad, a Deir al-Zor resident who did not want to give his full name.

He said at least 65 people had been killed since tanks and armoured vehicles barrelled into the provincial capital, 400 km (250 miles) northeast of Damascus, on Sunday, crumpling makeshift barricades and opening fire.

Later on Monday Assad fired defence minister Ali Habib and replaced him with chief of staff General Daoud Rajha. The state news agency said Habib was ill. Habib had been added to an EU sanctions list last week for his role in crushing protests.

The sudden withdrawal of ambassadors of Gulf Arab states leaves Assad with few diplomatic friends. Western states have imposed sanctions on top Syrian officials and countries with close ties to Damascus such as Russia and Turkey had warned Assad he was running out of time.

Nevertheless, countries have not proposed military action like that ranged against Libya's leader Muammar Gaddafi.

The Saudi criticism was the sharpest the oil giant has directed against any fellow Arab state since pro-democracy uprisings began to sweep across the region in January, toppling autocrats in Tunisia and Egypt, kindling civil war in Libya and rattling entrenched elites throughout the region.

"What is happening in Syria is not acceptable for Saudi Arabia," Abdullah said in a written statement read out on Al Arabiya satellite television.

"Syria should think wisely before it's too late and issue and enact reforms that are not merely promises but actual reforms," said the Saudi king, an absolute ruler whose country has no elected parliament. "Either it chooses wisdom on its own or it will be pulled down into the depths of turmoil and loss."

SHELLING AND GUNFIRE

The assault on Deir al-Zor, in an oil-producing province bordering Iraq, took place a week after tanks stormed the city of Hama, where residents say scores have been killed.

The official SANA news agency said on Monday the military was winding down there.

"Army units assigned to restore security and stability... have started to leave the city after they fulfilled their duty," it said. "Life is gradually returning to normal."

But an activist in Hama said there were still tanks in parts of the city and security forces were making arrests.

The Arab League also called for an end to the bloodshed. But its chief said on Monday it would use persuasion rather than "drastic measures" to resolve the conflict. Kuwait ruled out military action against Assad.

Those cautious responses contrasted with Arab League endorsement of the "no-fly zone" over Libya being enforced by NATO warplanes to support rebels fighting Gaddafi.

France repeated a call for Assad to scrap the military campaign which rights groups say has killed 1,600 civilians.

"The time of impunity is over for the Syrian authorities. This large-scale and bloody repression must stop," French foreign ministry spokeswoman Christine Fages said. Germany said Assad would lose his legitimacy if he did not stop bloodshed.

Saudi Arabia's decision to get behind the diplomatic pressure on Syria was unlikely to deter Assad, who has described the clampdown as a national duty, regional experts said.

"There is no evidence that outside statements or pressure in terms of sanctions has any impact on a regime in terms of policies," Beirut-based Middle East analyst Rami Khouri said.

Relations between Sunni Saudi Arabia and a Syrian ruling elite from Assad's minority Alawite sect have been tense since the assassination in 2005 of Rafik al-Hariri, a Western-backed Lebanese Sunni statesman who also had Saudi nationality.

Hama and Deir al-Zor are both predominately Sunni cities, and the crackdowns there resonate with Sunnis, who form the majority in the region and rule most Arab countries.

In Cairo, the head of the most influential school of Sunni Islam, al-Azhar, described the violence as a human tragedy that had to stop. "Blood only fuels the fires of revolutions," said Grand Imam Ahmed el-Tayeb.

Hama is known throughout the region as the site of a crackdown by Assad's father nearly 30 years ago against Sunni Islamists in which many thousands died.

AUTHORITIES DENY CITY ATTACKED

Syrian authorities denied that any Deir al-Zor assault had taken place. The official state news agency said "not a single tank has entered Deir al-Zor" and reports of tanks in the city were "the work of provocateur satellite channels."

Syria has barred most journalists, making it hard to confirm events reported by either side in the conflict.

"The Deir al-Zor assault could be the turning point where the repression will backfire and people will start taking up arms against the regime," one activist said. "Assad cannot repress a whole nation like this, and expect people to watch as thousands get killed or disappear."

Syrian authorities say they have faced attacks since the protests first erupted in March, blaming armed saboteurs for most of the civilian deaths and accusing them of killing 500 security personnel.

State television broadcast footage on Sunday of mutilated bodies floating in the Orontes river in Hama, saying 17 police had been ambushed and killed in the central Syrian city.

Turkish Prime Minister Tayyip Erdogan, who cultivated close ties with Assad but has sharply criticised the crackdown, said Foreign Minister Ahmet Davutoglu would visit Syria on Tuesday.

"Our message will be decisively delivered," he said, drawing a rebuke from an Assad adviser, who described the Turkish statement as unbalanced.

U.S. Secretary of State Hillary Clinton spoke to Davutoglu on Sunday, the State Department said, asking him to "reinforce" Washington's position that Syria must immediately return its military to barracks and release prisoners.