KABUL — The United States has cancelled funding for opinion polls in the run-up to Afghanistan's presidential election, a U.S.-funded group said, after a first poll in December triggered accusations of U.S. attempts to manipulate the outcome.

The cut in funding comes as relations between the United States and Afghanistan have been severely strained over President Hamid Karzai's refusal to sign a bilateral security pact that would enable U.S. troops to stay beyond this year.

An official at a U.S.-funded group that promotes democracy said it and other such organizations had planned to carry out opinion polls as Afghanistan prepares for the April 5 election.

"There were a few agencies that were responsible for conducting the polls but all of them have been cancelled at the moment," Mohammad Atta, program officer with the Democracy International group, said on Thursday.

His group had planned three rounds of opinion polls and published its first results in December. But its findings provoked a public outcry and accusations of interference.

The U.S. embassy declined to comment immediately on why the polls had been cancelled, but ties between the United States and Karzai, who has served two terms and is not running again, have been troubled for months.

Karzai has long suspected the United States of having interfered in the last presidential election in 2009 and has warned against further meddling.

Former U.S. defence secretary Robert Gates recently published a memoir appearing to confirm Karzai's suspicion, saying the then top U.S. envoy for Afghanistan and Pakistan had been "doing his best to bring about the defeat of Karzai".

Karzai's spokesman said that Washington may try to use polling as a means to influence the outcome of the elections this year.

"It is now crystal clear that there was interference in the election in 2009," said Aimal Faizi. "It puts the U.S. role behind such funding under question. Why would the U.S. fund surveys on Afghan presidential candidates?"

The United States is Afghanistan's biggest aid donor despite the difficult relationship with Karzai but it is committing about 15 percent less for the 2014 election fund than it did in 2009, pledging $55 million.

Democracy International's first poll showed the front-runners to be West-leaning intellectual and former finance minister Ashraf Ghani and former foreign minister Abdullah Abdullah, Karzai's main challenger in 2009.

The election is seen as crucial to Afghanistan's efforts to build stability, months before most foreign forces are due to withdraw, leaving Afghan forces to battle a resilient Taliban insurgency.

The United States has threatened to pull out all of its troops unless Karzai signs the security deal promptly.