US Secretary of State John Kerry said there had been no indication that the UK had purchased the weapons, despite speculation by UK Prime Minister Theresa May’s government.
Mr Kerry said that the US would look into it “very carefully” but that the British government had “no reason to doubt” that they had been supplied.
The UK has said it will hold a parliamentary inquiry into its purchase of British-made M249 assault rifles.
The US has said that its forces had no need for them.
US Secretary State John Dempsey said that it was “very troubling” that the sale of the M249s had been discussed.
He said the US was “not aware” of any British purchases of them.
UK Prime Minster Theresa May has denied any knowledge of any UK weapons sale to the US.
“We don’t believe there has been any discussion of weapons,” she told the BBC’s Andrew Marr Show.
“I’m sure we will have more information tomorrow.”
The UK Foreign Office said: “The US has made clear it has no intention of providing any military equipment to the UK, including to our troops.
We will look at all the evidence we can to support our view that there has not been a UK purchase of M249 rifles.”
The Foreign Office has said the UK will hold an inquiry into the purchase of its weapons.
It said it would be “very careful about what we say about what has happened” but would not comment further.
“The UK has a long history of contributing to peacekeeping missions abroad, including the NATO-led Resolute Support mission in Kosovo and the NATO Support Mission in Somalia, both of which were led by UK and American troops,” it said.
“Any such discussion would have to be carried out with full respect for the rights of the British people to know the facts.”
US Secretary-General John Kerry arrives at the United Nations headquarters in New York (AFP) Mr Kerry made the comments in an interview with NBC News.
He did not name the UK or discuss the British weapons sales, but said the British arms deal “has nothing to do with me”.
Mr Kerry also told the broadcaster that Britain would “take any step that we can” to ensure the UK does not “receive anything that could undermine our resolve”.
UK Foreign Secretary Boris Johnson has called on the UK government to “review” its ties with the US, which he said could be an opportunity to strengthen UK-US relations.
“There is no way that Britain can afford to be dependent on the US,” he told BBC Radio 4’s Today programme.
“It has no credibility at all in terms of our relationship.”
Mr Johnson said that in the past few weeks the UK has been receiving “very little information about these arms purchases”.
He said: We have received a lot of very negative comments about the British military and we know that there are people who are trying to undermine it.
“And that is what we want to do is to ensure that those comments are taken very seriously and that there is not any negative reaction.”
But the UK Foreign Ministry said it had not received any information from the US on the weapons purchase.
“British Defence has no role in the US-led mission in Iraq,” a spokesperson said.
British Prime Minister David Cameron has previously said that Britain was not buying US weapons.
“No-one in Britain has ever asked us to do anything, but it is absolutely clear that we have to do it.
We are absolutely committed to doing everything we can for the troops on the ground in Iraq and Syria,” he said.
US President Donald Trump said the purchase had been made in a bid to reassure the US of its “special relationship” with the UK.
“What happened yesterday is completely unacceptable and completely unacceptable to our allies,” he added.
“Britain has a special relationship with the United States, a close and special relationship.
And I have to tell you, the United Kingdom is one of the strongest countries in the world.”
“We are going to get along with anybody.
And we are going have a great relationship.”