According to US and European officials in Washington on Sunday, Secretary of State Condoleezza Rice and other officials told a Brussels delegation that any kind of cooperation agreement would be premature.
"We don't think it's a particularly good idea," a US official said summarising Rice's message to the EU leaders.
The unnamed official told Reuters that Rice raised objections because of the "whole pattern of Syrian behaviour", including what the official called Damascus's support for anti-Israeli groups, its failure to patrol the Iraq border, and US doubts about whether the Syrians have pulled "their covert presence" out of Lebanon.
Another unnamed European diplomat involved in the discussions also told Reuters that the Americans made clear they "don't want any positive gestures" made towards Syria at this time.
The latest move underscores US determination to maintain pressure on Damascus and keep a lid on any overtures from Europe to warm ties with Syria.
However, some European diplomats said the bloc was preparing to push ahead with signing the so-called Association Agreement as a reward to Syria for pulling its soldiers out of Lebanon after 29 years.
The US has doubts that Syria has
its 'covert presence' in Lebanon
The Association Agreement, which the EU and Syria initialled in October after months of wrangling over the wording of a clause renouncing weapons of mass destruction, would give the Arab state greater access to EU markets in exchange for progress in areas such as democracy and human rights.
Syria is the only country in the Euro-Mediterranean Partnership, which brings together 25 EU countries and their 12 Mediterranean neighbours, yet to sign such an agreement.
Damascus had hoped the EU pact would counterbalance economic sanctions imposed last year by Washington and denies allowing fighters to cross the border into Iraq.
And the UN confirmed that Syria has pulled out completely from Lebanon in May, though Washington remains sceptical.