Kofi Annan arrived in Damascus on Sunday evening for talks with President Bashar al-Assad, his spokesman said, a day after the international peace envoy admitted that his peace plan had so far failed to end 16 months of bloodshed.
UN and government sources said Mr. Annan and Mr. al-Assad would not meet tonight.
Also, Syria's navy fired live missiles from ships and helicopters over the weekend, state media said on Sunday, in an exercise aiming at demonstrating its ability to "defend Syria's shores against any possible aggression".
U.S. Secretary of State Hillary Clinton said Syrian opposition forces were growing more effective and the sooner the violence ended, the better were the chances of sparing Syria's government from a "catastrophic assault" by rebel fighters.
Syrian television aired video of a variety of missiles being fired from launchers on land and from ships and showed Syrian Defence Minister Dawud Abdallah Rahijia in attendance.
"Naval Forces conducted an operational live fire exercise on Saturday, using missiles launched from the sea and coast, helicopters and missile boats, simulating a scenario of repelling a sudden attack from the sea," Syrian news agency SANA said, adding that manoeuvres would continue for several days.
Opposition figures have been calling for a no-fly zone and NATO strikes against Syrian forces, similar to those carried out in Libya last year which enabled rebel ground forces to end the rule of Moammar Gaddafi.
But while Mr. al-Assad has faced sanctions and international condemnation over his crackdown on dissent which has left thousands dead, major Western and Arab powers have shied away from direct military action.
Turkey has reinforced its border and scrambled fighter aircraft several times since Syria shot down a Turkish reconnaissance jet on June 22 over what Damascus said were Syrian territorial waters in the Mediterranean. Ankara said the incident occurred in international air space.
"The sooner there can be an end to the violence and a beginning of a political transition process, not only will fewer people die, but there is a chance to save the Syrian state from a catastrophic assault that would be very dangerous not only to Syria but to the region," Ms. Clinton told a Tokyo news conference.
Ms. Clinton appeared to be referring to the possibility of Syrian rebels launching such an assault on state institutions rather than to any outside intervention.
"There is no doubt that the opposition is getting more effective in their defence of themselves and in going on the offence against the Syrian military and the Syrian government's militias. So, the future .... should be abundantly clear to those who support the Assad regime," Ms. Clinton added.
"The sand is running out of the hour glass."
More than 30 people were killed on Sunday during a government bombardment and clashes between Syrian forces and Free Syrian Army rebels fighting to oust Mr. al-Assad, the Syrian Observatory for Human Rights said.
Activists reported heavy shelling in residential areas of Deir al-Zor city and in Deraa province, the birthplace of the revolt near the Jordanian border.
Rami Abdul-Rahman, head of the Observatory, said residents of al-Sharifa in the wider Deir al-Zor province were reporting that rebels had for the first time taken over a tank and were using it to attack army positions.
The rebels have gained confidence in recent weeks, staging bolder attacks, holding pockets of territory across the country and clashing with troops only a few miles from the presidential palace in Damascus.
Mr. Annan arrived with deputy Syrian Foreign Minister Faisal Mekdad at the Dama Rose hotel in the capital, where United Nations observers have been staying since suspending their patrols because of a steep increase in the level of violence.
"Clearly, we have not succeeded. And maybe there is no guarantee that we will succeed," Mr. Annan told the French daily le Monde in an interview published on Saturday.
He said that Western criticism of Russia was diverting attention from the role of other countries in backing Mr. al-Assad and arming his soldiers, notably Iran. Mr. al-Assad has been Shi'ite Iran's main ally in the Arab world.
"Russia has influence, but I don't think that events will be determined by Russia alone. What strikes me is that there is so much talk about Russia and much less about Iran, and little is said about other countries that are sending money and weapons," Mr. Annan said.
Sunni Gulf states Saudi Arabia and Qatar have called for the arming and funding of the rebel forces.
"All these countries say they want a peaceful solution, but they undertake individual and collective actions that undermine the very meaning of [UN] Security Council resolutions," he added.