Some 1,000 workers on a project to expand the Panama Canal were on strike Wednesday to demand better safety and treatment, the latest setback for the behind-schedule, over-budget upgrade.

Union leaders said three workers had received electric shocks recently because their managers had forced them to work in the rain.

The strike interrupted the fourth phase of dry excavation on the canal's Pacific sector.

The Panama Canal Authority said it was "closely following these events, which represent a threat to the continuation of work in one sector of the canal expansion project."

It said the consortium carrying out the project had assured authorities all work was being carried out according to Panamanian labor law under the strict supervision of the labor ministry.

Workers blocked the access road to the key Centenary Bridge across the canal and vowed to stay put until the government answered their demands.

The Canal Authority urged both sides to find an agreement "to allow this project to proceed on schedule."

Work began in 2007 to expand the canal with a third set of locks to enable it to handle the modern mega-freighters that global shipping companies prefer.

But the $5.25 billion project has been plagued by delays, strikes and a bitter dispute over $1.6 billion in cost overruns with the consortium carrying out the upgrade, led by Spanish construction firm Sacyr.

Initially scheduled for completion in 2014, the project's due date has been pushed back to early 2016.

Nearby Nicaragua, meanwhile, launched construction this week on a rival canal, a $50 billion project that the Chinese firm behind it plans to complete in five years.