Commuter rail service will resume Friday following a strike, after the San Francisco Bay Area transit agency and its two largest unions agreed to extend a labor contract for a month while they continue bargaining.

A statement released late Thursday by Bay Area Rapid Transit said trains will begin running by 3 p.m. Friday. The agreement between the agency and its unions will be extended for 30 days.

BART General Manager Grace Crunican said there is a wide gap of disagreements between the two sides.

"Unfortunately, the issues that brought us to this point remain unresolved," Crunican said. "Despite lots of hard work, BART and its unions have failed to come to an agreement on contract issues that matter to all of us today and into the future."

BART is the nation's fifth-largest rail system and serves more than 400,000 commuters each weekday. BART carries passengers from the farthest reaches of San Francisco's densely populated eastern suburbs to San Francisco International Airport across the bay.

The strike began early Monday after negotiations broke off. Talks resumed Tuesday as political pressure and public pleas mounted to reach a settlement.

BART has said workers from the two unions average about $71,000 in base salary and $11,000 in overtime annually. The workers also pay a flat $92 monthly fee for health insurance.

The unions _ which represent nearly 2,400 train operators, station agents, mechanics, maintenance workers and professional staff _ want a 5 percent raise each year over the next three years.

BART said it is offering an 8 percent salary increase over the next four years as well as reducing the amount of employee contributions it originally requested for pension and medical benefits.

Antonette Bryant, president of one of the unions, said BART is on notice and has 30 days to hammer out a deal.

"We're not going to let them hijack us and the riding public and we are deeply sorry this had to happen," Bryant said.