KABUL, Aug 26 (Reuters) - The United States and allies urged people to move away from Kabul airport on Thursday due to the threat of an Islamic State terror attack as Western troops hurry to evacuate as many Afghans as possible before an Aug. 31 deadline.

Pressure to complete the evacuations of thousands of foreigners and Afghans who helped Western countries during the 20-year war against the Taliban has intensified with all U.S. and allied troops due to leave the airport next week.

In an alert issued on Wednesday evening, the U.S. Embassy in Kabul advised U.S. citizens to avoid travelling to the airport and said those already at the gates should leave immediately, citing unspecified "security threats".

Britain issued a similar advisory, telling people in the area of the airport to "move away to a safe location".

"There is an ongoing and high threat of terrorist attack", the British Foreign Office statement said. read more

Australia also urged its citizens and those with a visa for Australia to leave the area, warning of a "very high threat of a terrorist attack" at the airport.

The warnings came against a chaotic backdrop in the capital, Kabul, and its airport, where a massive airlift of foreign nationals and their families as well as some Afghans has been underway since the Taliban captured the city on Aug. 15.

While Western troops inside the airport worked feverishly to keep the evacuation moving as fast as possible, Taliban fighters guarded the perimeter outside where thousands of people have thronged, trying to flee the country rather than stay in a Taliban ruled Afghanistan.

"It's very easy for a suicide bomber to attack the corridors filled with people and warnings have been issued repeatedly," Ahmedullah Rafiqzai, an Afghan official working at the Directorate of Civil Aviation at the Kabul airport, told Reuters.

"But people don't want to move, its their determination to leave this country that they are not scared to even die, everyone is risking their life."

Taliban fighters have promised to provide security outside the airport, but intelligence reports of an imminent threat from Islamic State militants cannot be ignored, a NATO country diplomat in the Afghan capital said on Thursday.

The White House said President Joe Biden was briefed on Wednesday about the threat from the ISIS-K militant group as well as contingency plans for the evacuation.

Biden has ordered all troops out of Afghanistan by the end of the month, to comply with an agreement with the Taliban, despite European allies saying they needed more time to get people out.

In the 11 days since the Taliban swept into Kabul, the United States and its allies have mounted one of the biggest air evacuations in history, bringing out more than 88,000 people, including 19,000 in the past 24 hours. The U.S. military says planes are taking off the equivalent of every 39 minutes.