"There remain key differences between our two governments, including concerns about Syria's support for terrorist groups and networks, Syria's acquisition of nuclear and non-conventional weaponry, interference in Lebanon and a worsening human rights situation."

Duguid said the US expected the "mounting evidence and ongoing concerns" with Syria to be addressed by the International Atomic Energy Agency (IAEA) board of governors at its March 2 - 6 meeting.

"We fully support the IAEA in its investigation [into Syria's nuclear programme] and urge the international community to continue insisting that Syria comply with its IAEA obligations and cooperate fully with the IAEA without delay," he said.

Lebanese affairs

Relations between Syria and the United States nosedived after the 2005 assassination of Rafik al-Hariri, the former Lebanese prime minister, although Syria denies involvement.

Tensions have lingered over Damascus' support for the Palestinian group Hamas and the Lebanese Hezbollah movement, and accusations that Syria allowed fighters to infiltrate Iraq.

Bashar al-Assad, the Syrian president, told a British newspaper this week he hoped for better relations with the US and that Barack Obama, the US president, would send an ambassador to Syria soon.

The US withdrew its ambassador under in 2005 after al-Hariri's assassination.

John Kerry, chairman of the US senate foreign relations committee, will visit Syria over the weekend.