NASA said on Wednesday that it is being forced to extend its contract with Russia to fly astronauts to the International Space Station (ISS) due to “continued reductions in the president’s funding requests” for the agency.

The new contract, which runs through 2019 and will cost American taxpayers an additional $490 million, means that NASA would continue to depend on Russia to send astronauts to the ISS, even as Washington is imposing sanctions on the country in retaliation to its actions in eastern Ukraine. NASA Administrator Charles Bolden has, in the past, sparred with American lawmakers over funds allotted to NASA’s Earth science division.

“In 2010, I presented to Congress a plan to partner with American industry to return launches to the United States by 2015 if provided the requested level of funding,” Bolden said, in a letter sent to Congress Wednesday. “Unfortunately, for five years now, the Congress, while incrementally increasing annual funding, has not adequately funded the commercial crew program to return human spaceflight launches to American soil this year, as planned.”

Since the U.S. ended its space shuttle program in 2011, Russia’s Soyuz rockets are the only means of ferrying astronauts to and from the ISS. Under its current arrangement, NASA has to shell out nearly $80 million for every seat on the Soyuz rocket. 

Companies like Boeing Co and SpaceX are attempting to develop commercial spaceships to transport astronauts to and from the ISS by the end of 2017, which could end NASA’s dependence on Russia. Last September, NASA awarded SpaceX and Boeing $6.8 billion in contracts to develop their Dragon and CST-100 space capsule, respectively.

However, SpaceX’s recent attempts to launch resupply vehicles to the ISS have not been entirely successful. In June, a Falcon 9 rocket carrying crucial supplies of food, fuel, water and spare parts exploded shortly after launch from Cape Canaveral. SpaceX founder Elon Musk later attributed the failure of the mission to the company's “complacency” following a slew of successful launches.

NASA had hoped to start flying astronauts on U.S.-made rockets by 2017. However, reductions from its 2016 budget request for commercial crew -- part of House and Senate budget proposals for the fiscal year -- “would result in NASA’s inability to fund several planned CCtCap [Commercial Crew Transportation Capability] milestones in FY 2016 and would likely result in funds running out for both contractors during the spring/summer of FY 2016,” Bolden wrote, in the letter.

Although the White House has requested $1.24 billion for NASA’s commercial crew program for the next fiscal year, House Republicans have proposed allocating nearly $250 million less than the request, while Senate Republicans have offered $300 million less, according to media reports.

“While I understand that funding is extremely limited, it is critical that all of NASA’s human spaceflight efforts be supported,” Bolden wrote. “I urge Congress to provide the funds requested for our commercial crew program this year, so we can prevent this situation in the future.”