SAN SALVADOR, DEC. 12 -- Leftist rebels forced down a Salvadoran government helicopter today in a series of attacks that left at least 51 dead.

The armed forces said the heaviest combat was in the eastern provinces of San Miguel and Usulutan. Sporadic fighting continued throughout the day.

Honduras said about 200 Salvadoran soldiers, some seriously wounded, crossed its border after a battle with the rebels on Salvadoran territory. The Honduran Foreign Ministry said U.N. military observers were sent to verify the incident and provide assistance. Rebels did not enter Honduras, it said. The incident was reported to the Salvadoran government, and the troops will be sent home, the ministry said.

The ministry said a senior official of the rebel Farabundo Marti National Liberation Front, Miguel Saenz, scheduled a meeting with Honduran President Rafael Callejas to discuss the incident.

Attacks began with a rebel assault Tuesday night on army positions on the fringes of San Jorge, 95 miles southeast of San Salvador. The army said 11 soldiers and 30 guerrillas died there.

The rebels' clandestine Radio Venceremos said guerrilla ground fire forced down a government UH-1H helicopter early today near the Honduran border. The armed forces said the copter was only slightly damaged but acknowledged that one gunner was killed and the pilot, copilot and a second gunner were wounded.

The FMLN also attacked an army garrison 45 miles north of the capital in Chalatenango. Radio Venceremos said FMLN fighters captured 11 soldiers nearby and gave the soldiers' names.

The FMLN began stepping up the pace of combat Nov. 20 in what it said was a campaign to force the right-wing government into concessions at the negotiating table and punish the armed forces for human rights abuses.