"Now is the entry, descent and splashdown phase after we undock, hopefully a little bit later today," Doug Hurley, one of the two astronauts, said in a farewell ceremony aboard the ISS that was broadcast on NASA TV.

"The teams are working really hard, especially with the dynamics of the weather over the next few days around Florida," he said.

"And we appreciate those efforts because I know those decisions and that work isn't easy."

Hurley and Bob Behnken 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.

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

"Conditions are 'Go' at the primary targeted site, off the coast (of) Pensacola, and alternate site off the coast of Panama City in the Gulf of Mexico for splashdown and recovery on Sunday, Aug 2," NASA said in a blog post.

But the US space agency is following the progression of Isaias, a category one hurricane, which was heading for Florida on Saturday after pummeling the Bahamas with heavy rain.

"NASA and SpaceX will make the final decision to proceed after the astronauts are ready inside Crew Dragon just before undocking," it said.

That call should come at about 2100 GMT.

"Teams will continue to closely monitor Hurricane Isaias and evaluate impacts to the potential splashdown sites," NASA added.

Earlier, during the ISS ceremony, Behnken said that "the hardest part was getting us launched. But the most important part is bringing us home."

Addressing his son and Hurley's son, he held up a toy dinosaur that the children chose to send on the mission and said: "Tremor The Apatosaurus is headed home soon and he'll be with your dads."

Behnken later tweeted: "All my bags are packed, I'm ready to go."

- 'Exciting day' -

Mission chief Chris Cassidy called it an "exciting day" and said he was sad to see the pair leave the ISS, hailing the importance of having a new means to transport astronauts.

The mission marked the first time a crewed spaceship had 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, US astronauts traveled exclusively on Russian Soyuz rockets, for a price of around $80 million per seat.