* Annan says talks with Assad positive, heads to Iran
* Assad says countries supporting rebels block peace plan
* Clinton says rebels on offensive, time running out
Kofi Annan said he had constructive talks in Damascus on Monday with Syrian President Bashar al-Assad, who said the U.N. peace envoy's plan to end 16 months of bloodshed was being undermined by U.S. political support for "terrorists".
Within hours, Annan was due to head for Iran for talks with Syria's main ally in the region.
"I just had a positive and constructive discussion with President Assad," the United Nations special envoy said.
"We agreed an approach which I will share with he opposition," he told reporters. Once again, Annan stressed the important of halting violence and promoting political dialogue -- the key points of the plan he put forward in April.
Syrian Foreign Ministry spokesman Jihad Makdissi said in a Twitter message: "In both meetings we reassured Annan of Syria's commitment to implement the 6-points Plan and hoped other side is mutually committed."
In a television interview aired on Sunday, Assad accused Saudi Arabia, Qatar and Turkey, as well as the United States, of supplying arms and logistical support to the rebels trying to overthrow him.
"We know that (Annan) is coming up against countless obstacles but his plan should not be allowed to fail, it is a very good plan," he told Germany's ARD network.
"The main obstacle (is) that many countries don't want (it) to succeed. So they offer political support and they still send armaments and send money to terrorists in Syria," Assad said, according to a transcript of the interview, held in English.
Syria, led by members of a sect related to Shi'ite Islam, has alleged that the Sunni-led Gulf monarchies are supporting unrest among its Sunni majority as a way to check rising Shi'ite influence in the region, most notably that of Shi'ite Iran.
Anti-Assad activists in Syria reported army shelling and clashes with rebels on Monday in Deir Ezzor, Deraa, Homs, Aleppo and a neighbourhood of Damascus. Residents also reported the sound of gunfire in the capital.
An activist website said over 100 Syrians had been killed on Sunday, most of them civilians.
U.S. Secretary of State Hillary Clinton said on Sunday that Syrian opposition forces were growing more effective and the sooner the violence ended, the better the chances of sparing Syria's government a "catastrophic assault" by rebel fighters were.
While Assad has faced sanctions and international condemnation over his crackdown on dissent, 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.
"SAND RUNNING OUT"
"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," Clinton told a Tokyo news conference.
She 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," Clinton added.
"The sand is running out of the hour glass."
Syria's navy fired live missiles from ships and helicopters over the weekend, in an exercise aimed at demonstrating its ability to "defend Syria's shores against any possible aggression", state media said.
Rami Abdelrahman, head of the Syrian Observatory for Human Rights, said residents of al-Sharifa in the wider Deir Ezzor province on Sunday reported that rebels had captured a working tank for the first time 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.