BEIRUT, FEB. 5 -- Hooded guerrillas kidnaped two United Nations relief workers near the southern port city of Sidon today, prompting a warning by a senior U.N. official here that the organization's international relief staff may be evacuated.

Four gunmen intercepted the car driven by the two relief workers, a Swede and a Norwegian, and forced the two into another vehicle before speeding off, police said. The workers' car bore diplomatic license plates and the blue-and-white flag of the U.N. Relief and Works Agency (UNRWA).

UNRWA's director for Lebanon, Per Olof Hallquist, said he was convinced the abductions were an error. "If events prove otherwise, however, we will have no choice but to consider a forced departure of all nonessential international staff from Lebanon as a matter of urgency," he warned.

The two kidnap victims, Jan Stening, 44, from Sweden, and William Jorgensen, 58, from Norway, have been working in Lebanon since last spring as supervisors for the southern Lebanon operations of UNRWA, which specializes in Palestinian refugee affairs.

The U.N. agency employs 2,200 people in Lebanon, including 14 from Norway, Sweden, Ireland, Australia, Japan, Yugoslavia and Finland. The agency helps administer schools and food programs for 278,000 Palestinian refugees here.

The abductions took place at Siddiqin, south of Sidon, in the vicinity of two checkpoints controlled by Shiite and Sunni Moslem militiamen. Militia officials said they believed the two were taken in the direction of the sprawling Ain Helweh Palestinian refugee camp.

Militia and security sources speculated that the Palestinian terrorist group Abu Nidal carried out the kidnaping because of recent disputes between the group and UNRWA concerning relief rations.

The Fatah Revolutionary Council, the formal name of the Abu Nidal organization, issued a statement in Sidon flatly denying that its men had carried out the kidnaping.

No group has claimed responsibility for the abductions, which came 10 days after a West German national was kidnaped in Moslem-controlled west Beirut by a pro-Iranian group, and three days after the mysterious assassination in Christian east Beirut of a Frenchman said to be a secret agent.