MOUNTAIN VIEW, Calif. — NASA announced Feb. 6 another set of delays to the schedule of commercial crew test flights by Boeing and SpaceX, increasing concerns that the vehicles won’t be ready to starting transporting astronauts to the International Space Station by the end of this year.

In a statement, NASA said that the initial uncrewed test flight by SpaceX’s Crew Dragon spacecraft, previously scheduled for no earlier Feb. 23, was now scheduled for no earlier than March 2. A second test flight, with NASA astronauts Bob Behnken and Doug Hurley on board, is now scheduled for July.

Boeing’s first uncrewed test flight of its CST-100 Starliner vehicle, previously scheduled for March, is now expected no earlier than April. A crewed test flight, with NASA astronauts Mike Fincke and Nicole Mann, and Boeing astronaut Chris Ferguson, is scheduled for no earlier than August.

The revised dates, NASA said in its statement, “allow for completion of necessary hardware testing, data verification, remaining NASA and provider reviews, as well as training of flight controllers and mission managers.” No specific technical issues were cited as reasons for the delays, nor was the five-week partial government shutdown in December and January.

“The uncrewed flight tests are a great dry run for not only our hardware, but for our team to get ready for our crewed flight tests,” Kathy Lueders, NASA’s commercial crew program manager, said in the statement. “NASA has been working together with SpaceX and Boeing to make sure we are ready to conduct these test flights and get ready to learn critical information that will further help us to fly our crews safely.”

The overall commercial crew program has suffered extensive delays. The date for SpaceX’s initial launch, also known as Demo-1, has slipped nearly two months, from Jan. 7 to March 2, since NASA announced that Jan. 7 launch date Nov. 21.

SpaceX, though, has been making progress, including rolling out the Crew Dragon spacecraft atop its Falcon 9 rocket to Launch Complex 39A at the Kennedy Space Center in early January for tests. The rocket performed a static fire test on the pad Jan. 24 that the company said was successful.

While the schedule for the crewed test flights are little changed from previous schedules, there’s an expectation in the industry that those dates, too, will slip depending on when the uncrewed test flights take place and their outcomes. That’s led to worries that neither vehicle may be certified to carry NASA astronauts by the end of 2019, when the agency’s access to seats on Russian Soyuz seats runs out.

For now, NASA continues to expect that at least one of the vehicles will be ready for operational missions by the end of this year. That public belief extends to the White House. President Trump made a passing reference to commercial crew missions in his State of the Union address Feb. 5. “This year, American astronauts will go back to space on American rockets,” he said, after mentioning the upcoming 50th anniversary of the Apollo 11 landing.