Syrian forces have dropped more than 2,000 barrel bombs across the country since July, killing hundreds of people, the US ambassador to the United Nations said on Thursday.

US envoy Samantha Power called for action to end the use of a type of improvised explosive that has particularly been targeted at the Damascus suburb of Darayya and the southwest region of Zabadani, near the Lebanon border.

"The Assad regime has apparently grown reliant on the repugnant use of barrel bombs as an instrument of terror against innocent Syrian civilians," Power said in a statement.

"It is long past time for the international community to come together to end the deplorable use of barrel bombs and all other forms of attacks against civilians in Syria."

The United States, France and Britain have repeatedly accused President Bashar al-Assad's forces of using helicopters to drop barrels rigged with explosives on civilian areas.

Power condemned the latest wave of indiscriminate bombings that have "killed hundreds of people and destroyed schools, mosques, markets, hospitals and ambulances".

The UN Security Council is discussing proposals for a resolution on barrel bombs that would increase the pressure on Damascus even though it adopted a resolution in February last year demanding an end to the attacks.

Human rights group say barrel bombs are the leading killer in the war, now in its fifth year, with more than 240,000 people dead.

Syrian opposition in Russia

Meanwhile, Syria's main opposition group on Thursday insisted that Assad must go as it met with Russia's foreign minister.

The head of Syria's National Coalition Khaled Khoja held talks with top diplomat Sergei Lavrov as part of a fresh push by Russia to find a way out of the four-year civil war.

Moscow - one of Assad's main backers - is pushing a plan for a broader grouping than the US-led coalition fighting the Islamic State (IS) group, to include Syria's government and its allies.

But Khoja - in Moscow for his first talks since February 2014 - ruled out cooperating with Assad and reiterated demands that the strongman must leave before any transitional government can be set up.

"Bashar Assad has no role in the future of Syria," Khoja said in an interview with the Interfax news agency translated into Russian.

At the start of the meeting, Lavrov insisted that Russia was working with regional and international players to find a political solution to the crisis and stop Syria from becoming a "hotbed of terrorism".

"The main thing now is that these interests translate into practical coordinated steps," Lavrov said.

National Coalition representative Badr Jamous described the visit as "very good," Russian Interfax reported after the sit-down.

"There were many issues where we agreed with the Russian representatives," Jamous was quoted as saying.

Saudi Foreign Minister Adel al-Jubeir rejected calls to work with Assad against IS after a meeting with Lavrov in Moscow on Tuesday.

The spate of meetings is part of a broader diplomatic flurry that saw Lavrov sit down with Jubeir and US Secretary of State John Kerry in Doha earlier this month.

As part of the push, Lavrov is expected to meet with the head of a newer grouping of opposition figures known as the Cairo Conference Committee on Friday.

On Wednesday, Russia's top Middle East envoy met in Moscow with the head of the Syrian Kurdish Democratic Union Party (PYD) Saleh Muslim to discuss the mooted anti-IS coalition and attempts to unite Syria's opposition groups.

Syria's opposition and Western officials have hinted that Moscow's backing for Assad may be wavering, but Moscow insists it remains firmly behind the Syrian leader.

Kerry complains to Moscow about Iran general's visit

Meanwhile, Kerry called Lavrov on Thursday to express concern about a visit to Moscow by the commander of Iran's covert forces, a senior State Department official said.

General Qassem Suleimani, head of the Iranian Revolutionary Guards Corps' foreign operations, reportedly visited Russia late last month despite being subject to UN-backed international sanctions.

A State Department spokesman said on Wednesday the United States had confirmed the trip had taken place and said US officials would raise their concerns with Russia at an upcoming New York meeting on violent extremism.

"Secretary Kerry also raised his concerns about the travel to Moscow by IRGC Commander Qassem Suleimani," the senior official said on Thursday, outlining a call between the two diplomats.

Suleimani is one of several Iranian officials targeted by a 2007 United Nations travel ban because of their alleged links to Iran's nuclear or ballistic missile programmes.

Despite the recent deal struck by Iran and world powers on its nuclear programme, the targeted sanctions against Suleimani and many of his colleagues remain in effect.

Suleimani has also been sighted visiting Iranian-backed forces in Iraq.