Syria has reportedly threatened to close its border with Lebanon if UN peacekeepers are deployed along it.
Finland's foreign minister outlined the Syrian position after meeting his Syrian counterpart in Helsinki.
"They will close their borders for all traffic in the event that UN troops are deployed..." Erkki Tuomioja said.
Earlier, the Syrian president, Bashar Assad, said the stationing of UN troops just across its border with Lebanon would be a hostile move against Syria.
"This is an infringement on Lebanese sovereignty and a hostile position," President Bashar Assad told Arab TV.
The comments came as Israel urged rapid action over an expanded peace force, warning of an "explosive" situation on the ground amid the diplomacy.
Israel accuses Syria of supplying arms to Hezbollah across the border with Lebanon, including the rockets which were used to attack Israel throughout the month-long conflict.
Efforts to build the expanded 15,000-strong UN force for Lebanon have been dogged by delay and difficulty.
The UN has been disappointed by the response so far from European nations, and says a bolstered force is urgently needed to enforce the fragile truce.
Time was running out for the UN ceasefire resolution to be applied, Israeli Foreign Minister Tzipi Livni said after talks in Paris on Wednesday.
'State of enmity'
The BBC's Michael Voss, in Damascus, says Lebanon once again finds itself caught between the Israelis and the Syrians.
Israel has indicated it will not lift the air and sea blockade on Lebanon until international peacekeepers take up positions along the border.
But, in an interview with Dubai Television, Mr Assad said: "This means creating a state of enmity between Syria and Lebanon.
"First, it robs Lebanon of its sovereignty. No single state in the entire world would tolerate deploying foreign troops on its border posts unless there is a state of war with the other state...
"The second point is that it signals a hostile stance towards Syria. Naturally, it will create problems between Syria and Lebanon."
Slow diplomatic progress
Mr Tuomioja, whose country holds the EU presidency, will visit counterparts in Berlin and Paris on Thursday to discuss the bloc's contribution to the UN peacekeeping force.
And UN secretary-general Kofi Annan will have talks in Europe on Friday before heading for the Middle East, officials said on Wednesday.
The UN has been disappointed by the response so far from European nations over the creation of the bolstered peace force urgently needed to enforce the fragile truce.
Many nations have been hesitant to commit troops until there is greater clarity about the force's mandate, particularly on the issue of disarming Hezbollah.
Ms Livni echoed the sense of urgency after her talks in France, which has offered only 200 extra personnel for the peace force.
"Time is working against those who would like to see this resolution applied," Ms Livni said.
"We are now in the most sensitive and explosive position."
The 10-day-old truce has already been tested by a number of skirmishes and an Israeli commando raid deep inside Lebanon.
Since the truce came into effect, Israel has maintained restrictions on air and sea access to Lebanon, bringing a plea from Lebanese Prime Minister Fouad Siniora for US intervention.
"The United States can support us in putting real pressure on Israel to lift the siege," he told reporters on Wednesday.