Kofi Annan said he had constructive talks in Damascus on Monday, July 9 with Syrian President Bashar al-Assad, who said the UN peace envoy's plan to end 16 months of bloodshed was being undermined by U.S. political support for "terrorists," Reuters said.

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 the 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.

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.

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.