Bob Behnken and Doug Hurley blasted off from Cape Canaveral on May 30 on board a SpaceX Crew Dragon, and are supposed to splash down off the coast of Florida on Sunday afternoon.

For now, undocking remains scheduled for approximately 7:34 pm (2334 GMT) Saturday, and splashdown at 2:42 pm (1842 GMT) on Sunday.

But NASA said it was keeping a watchful eye on Hurricane Isaias -- a category one storm that battered the Bahamas Friday and was churning toward Florida -- and would make a final call about six hours prior to undocking.

"We don't control the weather, and we know we can stay up here longer -- there's more chow, and I know the space station program has more work that we can do," Behnken told reporters in a press call.

The potential splashdown sites are in the Gulf of Mexico and along Florida's Atlantic coast.

The mission marked the first time a crewed spaceship launched into orbit from American soil since 2011 when the Space Shuttle program ended.

It was also the first time a private company has flown to the ISS carrying astronauts.

The US has paid SpaceX and aerospace giant Boeing a total of about $7 billion for their "space taxi" contracts.

But Boeing's program has floundered badly after a failed test run late last year, which left SpaceX, a company founded only in 2002, as clear frontrunner.

For the past nine years, American astronauts traveled exclusively on Russian rockets Soyuz rockets, for a price of around $80 million per seat.

The crewmates added they were looking forward to going home after two months, if the return trip went ahead Sunday as planned.

"My son is six years old and I can tell from the videos that I get and from talking to him on the phone that he's changed a lot," said Behnken.