WASHINGTON - US President Barack Obama and British Prime Minister David Cameron on Tuesday discussed a number of hot-button issues including Syria, Afghanistan and Iran.

In their video conference, the two leaders expressed "concern" about the continuing violence in Syria, where conflict between the government and opposition forces have entered its 21st month without signs of abating.

Obama warned once again the Syrian government against using chemical weapons in the conflict and urged the government to secure the deadly weapons, the White House said.

On Afghanistan, the two leaders focused on negotiations of a US-Afghanistan bilateral security agreement, as American and NATO forces will withdraw most of their combat troops from the Asian nation by the end of 2014.

"They agreed on the need to find a way forward on Middle East peace that stops the cycle of counterproductive unilateral actions by the Israelis and Palestinians, and brings the parties back to the negotiating table," the White House said in a statement.

The Palestinians won an upgraded status of non-member observer state through the UN General Assembly late last month despite vehement opposition from the US and Israel, while Israel has reacted by unveiling plans to build more houses in settlements in east Jerusalem and the West Bank, making the prospect of renewed direct talks even more remote.

"On Iran, the two leaders agreed on the need to continue implementing sanctions, which are having an important effect, while pursuing diplomatic engagement to resolve the international community's concerns about Iran's nuclear program," the White House said.

The Obama administration set a March deadline late last month for Iran to begin "substantive cooperation" with the International Atomic Energy Agency (IAEA) over its nuclear program or face the UN Security Council.

Iran had one-day talks last week with IAEA experts in its capital of Tehran, the first since August, which the Islamic republic described as "constructive".

Cameron also expressed his condolences over the deadly shooting at Sandy Hook Elementary School in Newtown, Connecticut on Friday.

Adam Lanza, 20-year-old, shot and killed his mother Nancy on Friday, and broke into Sandy Hook Elementary where he killed 20 children and 6 adults before committing suicide.