A Republican congressman at the forefront of hearings into the killing of the US ambassador to Libya has attempted to directly tie the White House to security decisions that may have contributed to the disaster.

Jason Chaffetz has alleged that there was a "co-ordinated effort" between the White House and the state department to scale down security at US diplomatic missions in Libya in the months before a militia attacked the American consulate in Benghazi on September 11, killing the ambassador, Chris Stevens, and three other officials.

Chaffetz's allegation comes ahead of a hearing by the House of Representatives oversight committee on Wednesday into whether the state department contributed to the disaster by failing to heed warnings from Stevens and its own security personnel of the scale of the threat.

US state department officials on Tuesday gave their most detailed description yet of the events in Benghazi, and backed away from earlier assertions that the incident was triggered by protests against an anti-Islam video.

They described frantic and prolonged efforts to rescue the ambassador, Chris Stevens, from a smoke-filled "safe haven" inside the US diplomatic compound in Benghazi where he apparently died of asphyxiation.

The officials said there was nothing unusual happening near the Benghazi mission before the assault. Earlier accounts by White House and state department officials, including Susan Rice, the US ambassador to the United Nations, suggested that the attacks were triggered by protests over a video made in California that insulted the prophet Muhammad.

Last week, Chaffetz, chairman of a subcommittee on national security, and congressman Darrell Issa, chairman of the oversight committee, wrote to Hillary Clinton, the secretary of state, accusing the administration of incompetence and covering up the true nature of what they say was a well-planned terrorist assault with links to al-Qaida.

Now, in an attempt to pin ultimate responsibility on Barack Obama, Chaffetz says those cuts were at the behest of the White House.

"It seems to be a coordinated effort between the White House and the state department, from Secretary Clinton to President Obama's White House," Chaffetz told Fox News. "There was a very conscious decision made. My personal opinion is that they wanted the appearance of normalisation there in Libya, and [that] putting up barbed wire on our facility would lead to the wrong impression. Something that this administration didn't want to have moving forward."

However, Chaffetz admitted he had no evidence for his assertion.

Chaffetz and Issa are also vulnerable to accusations of hypocrisy after it was revealed that both congressmen voted to cut the state department's diplomatic security budget after Republicans took control of the House of Representatives in 2010. Republicans have shaved nearly $800m off state department spending on worldwide security over the past two years including $376m cut from embassy protection this year alone.

Among those called to testify before the oversight committee is the former head of US security in Libya, lieutenant colonel Andy Wood. He told CBS news that American officials, including himself and Stevens, made repeated calls for more security personnel in the months prior to the attack but the state department cut back instead.

"We tried to illustrate … to show them how dangerous and how volatile and just unpredictable that whole environment was over there. So to decrease security in the face of that really is … it's just unbelievable," Wood said.

He told CBS that when he was told that his 16-member team and a separate state department elite force of six were being withdrawn from Libya in August he felt "like we were being asked to play the piano with two fingers. There was concern amongst the entire embassy staff".

"They asked if we were safe," he said. "They asked … what was going to happen, and I could only answer that what we were being told is that they're working on it – they'll get us more (security personnel), but I never saw that."

The state department denies Wood's claim, saying there was no reduction in security personnel in the weeks before the attack in Benghazi. It said that when Wood's unit was withdrawn it was replaced bother others and that the number of security personnel in Libya remained constant.

The state department also said Wood was based in Tripoli and had no knowledge of the situation in Benghazi, 400 miles away.

The US under-secretary of state for management, Patrick Kennedy, will be the highest state department official to testify.