Doug Hurley and Bob Behnken, the first US astronauts to reach ISS on an American spacecraft in nearly a decade, will be closing out two months aboard the orbiting laboratory, with their capsule set to undock around 2334 GMT.

"The @SpaceX Crew Dragon forward hatch has been sealed for departure," the US space agency wrote on Twitter, adding that the astronauts were to set depart at approximately 2334 GMT.

The pair blasted off from Cape Canaveral on May 30 and are set to splash down on Sunday.

Their proposed splash-down sites are on the western side of Florida, while tropical storm Isaias's path has it headed near the east coast of the state.

"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.

Still, the US space agency is following the progression of Isaias, which was downgraded to a tropical storm from a hurricane Saturday evening. It was churning toward Florida after pummeling the Bahamas with heavy rain.

"Now is the entry, descent and splashdown phase after we undock, hopefully a little bit later today," Hurley 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.

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.