The Dragon spacecraft, owned by US company SpaceX, could launch no earlier than March 20 but a more fixed date would follow in the next couple of weeks, Nasa spokesperson Mike Suffredini told reporters.
The launch, the first-ever bid by a private company to dock at the orbiting lab, had been set for February but was postponed for technical reasons.
SpaceX - owned by Elon Musk, an internet entrepreneur and founder of PayPal - made history with its Dragon launch in December 2010, becoming the first commercial outfit to send a spacecraft into orbit and back.
SpaceX and several other companies are competing to build and operate a private capsule that could tote astronauts and cargo to the ISS, after US space agency Nasa retired its space shuttle program last year.
The main goals of SpaceX's next flight include a fly-by of the ISS at a distance of 3km and a berthing operation in which the Dragon will approach the ISS and the crew aboard the orbiting outpost will use the ISS robotic arm to help it latch on.
After the test docking, the Dragon aims to detach from the station for its return to Earth and eventually splash down in the Pacific off the coast of California.