DAMASCUS, Syria - Leaders of France, Britain and the United States have held phone conversations to discuss how to provide further support for Syria's opposition, which is fighting an increasingly fierce war with government forces.

A statement by the White House on Wednesday said a phone call between President Barack Obama and British Prime Minister David Cameron covered a "wide array of global issues," including the conflict in Syria and the need for increased participation from other countries to support the Syrian opposition.

The two leaders exchanged views on "ways the international community can assist those displaced by the conflict, apply pressure on the Assad regime, and support the opposition so that the legitimate aspirations of the Syrian people can be realized," the statement said.

Meanwhile, a statement from Cameron's office said the two leaders agreed that the use or threat of use of chemical weapons by Syria was "completely unacceptable" and would force them to "revisit their approach" to the conflict.

"As with (French President) Hollande, the prime minister and Obama discussed how to build on the support already given to the opposition to end the appalling violence in Syria and bring about stability," the statement said.

Western countries and some of its Arab allies have agreed in early July at a "Friends of Syria" meeting to "greatly increase assistance to the opposition" by giving them tools to communicate more securely with each other and the outside world.

Some of the Arab countries have also been reportedly providing weaponry to Syria's rebels. All the countries accused of arming the opposition by the Syrian government have so far denied the allegation.

Also on Wednesday, the Russian Foreign Ministry accused Western countries of engaging in "open incitement" of the Syrian opposition.

"Our Western partners still have done nothing to influence the opposition and to encourage it for dialogue with the government. Instead, they are engaged in open incitement to continue the armed struggle," the Foreign Ministry said in a statement.

Moscow has received this week a delegation headed by Syria's Deputy Prime Minister Qadri Jamil, who had visited the Russian capital only two weeks before.

In another related development, Iraq on Wednesday closed a major border checkpoint with Syria in its western Anbar province.

"Iraq closed the al-Qaim checkpoint early morning Wednesday with a three-meter concrete wall blocking the entry while forces of the Syrian opposition still hold control of the other side of the checkpoint," the source told Xinhua on condition of anonymity.

The source did not specify why the checkpoint was closed and when it would be re-opened.

The protracted Syrian crisis has killed at least 18,000 people, according to UN figures.

The escalating violence has also made thousands flee to neighboring countries, and for those displaced inside Syria, about 2.5 million are in desperate need of aid, a UN official said.

"This conflict has taken on a particularly brutal and violent character," UN aid chief Valerie Amos told a news conference in New York on Wednesday after visiting Syria and Lebanon last week.

"We face problems with access to people in need, particularly where there is intense and ongoing fighting, but funding is also holding us back. If we had more resources, we could reach more people," she said.