UNITED NATIONS (Reuters) - UN cartographers are working at "full speed" to define where the disputed Shebaa Farms between Syria and Lebanon begin and end and which country has jurisdiction, a UN envoy said on Monday. But Terje Roed-Larsen, the top diplomat on Syria and Lebanon relations, told reporters there was "no valid and acceptable geographical definition of what the area is."

"This is why UN cartographers are now working full speed on analyzing if it is at all possible to define in geographical and territorial terms what this area actually encompasses," Roed-Larsen said.

The United Nations says the Shebaa Farms is Syrian territory captured by Israel in the Middle East war of 1967. Syria and Lebanon say it is Lebanese and the Lebanese villagers show visitors their deeds to land in the area.

But Damascus has not signed an agreement with Beirut to demarcate the frontier, which Lebanon's Hizbollah militia has used as a "resistance" front to end Israeli occupation there.

However, the UN Security Council resolution in August that imposed a cease-fire on the Hizbollah-Israeli war asks Syria to sign with Lebanon a clear border agreement. Beirut has proposed that the Shebaa Farms should temporarily become an enclave controlled by the UN peacekeepers until a border agreement is reached.

"If we are going to ask, hypothetically, Israel to withdraw from this territory, we have to know what it is," Roed-Larsen said. The Shebaa Farms area is about 15 square miles.

In September, Israeli Prime Minister Ehud Olmert said Israel would be willing to discuss the status of the Shebaa Farms, but only if Lebanon disarms Hizbollah.

Olmert made the remarks during talks with visiting Russian Foreign Minister Sergei Lavrov, his office said.

Roed-Larsen said he had no date to complete the work as the United Nations had to analyze and translate hundreds of pages of documents received from Lebanon.