A huge fire erupted at a tire dump adjacent to the Beirut seaport Thursday, sending plumes of black smoke over large areas and burning relief supplies in a U.N. warehouse, police and a U.N. spokeswoman said.

Firefighters struggled to put out the blaze and prevent it from reaching one of the docks holding relief aid shipments sent to help Lebanese after a 34-day fighting between Israeli forces and Hezbollah guerrillas.

CountryWatch: Lebanon

But a warehouse run by the U.N. refugee agency caught fire, and a large number of blankets, tents and mattresses inside were burned, a UNHCR spokeswoman said.

"We are now assessing what the damage is," said the spokeswoman, Astrid Van Stort.

Van Stort and Lebanese officials said they did not know yet what sparked the blaze.

Click here to visit FOXNews.com's Mideast Center.