UPDATE 2-Third oil-capture vessel partially hooked up

* Third vessel partly hooked up, rough seas hinder finish

* Relief wells on target for August finish - commander (Recasts; adds details, quotes from briefing)

By Kristen Hays

HOUSTON, July 6 (Reuters) - A third vessel that will nearly double BP's (BP.L: Quote) (BP.N: Quote) oil-capture capacity to 53,000 barrels a day at its blown-out Gulf of Mexico was partially hooked up on Tuesday, but rough seas hampered efforts to finish the job, the top U.S. official overseeing the spill response said.

Retired Coast Guard Admiral Thad Allen said that when that vessel is hooked-up and siphoning oil, government officials and BP will decide in seven to 10 days whether to move to a four-vessel system that can capture up to 80,000 barrels a day.

"It is weather-dependent," he said of the hookup of the third vessel, which will join a drillship and a rig that collected or burned 24,980 barrels of oil on Monday, according to BP figures. "This is a work in progress."

Allen answered questions during a visit to BP's U.S. headquarters in Houston, where he said he was meeting with company officials to discuss the oil-capture systems.

He knocked down talk the first of two relief wells, now less than 300 feet from the bottom of the gushing well, would intercept it and plug the leak in July, up to two weeks early.

Early to mid-August remains the target finish date for both wells, Allen said. Current drilling is slow, with stops and starts, to allow BP to use sensors to ensure the relief well is on the right path, he said.