The alert is the most specific yet by Western intelligence but comes after weeks of warnings that the games could be targeted by al-Qaeda or Islamist separatists
Terrorists may be plotting to bring down airliners heading for the Sochi Olympics using explosives hidden inside toothpaste tubes, US intelligence has warned.
The Department of Homeland Security (DHS) issued the caution to airlines less than 48 hours before Friday's opening ceremony in the troubled region of southern Russia.
The alert is the most specific yet by Western intelligence but comes after weeks of warnings that the games could be targeted by al-Qaeda or Islamist separatists seeking independence from Russia.
"Our security apparatus includes a number of measures, both seen and unseen, and DHS will continue to adjust security measures to fit an ever evolving threat environment," a US official said.
While airlines normally allow passengers to carry liquids in containers smaller than 100ml, Russia has banned any liquids on planes heading into Sochi.
In August 2004, two Russian passenger jets were simultaneously destroyed by suicide bombers, thought to have hidden their explosives inside containers of face cream.
Rules on liquids were introduced after British intelligence disrupted a 2006 plot to destroy 10 planes flying from Heathrow using liquid explosives.
The Russian government has mounted an unprecedented security operation at the Winter Olympics, which will last from Friday until February 23.
Vladimir Putin, the Russian president, has invested his personal authority in the success of the games as a way of demonstrating that his country is safe.
Yet athletes and spectators remain anxious following three suicide bombings in the last three months on targets around Sochi.
Peter King, a Republican congressman who sits on the homeland security committee, said that Russian intelligence had not been as cooperative ahead of the games as the US would like.
"I have some confidence in what the Russians are doing but really not enough," he told CNN, adding that he would not feel safe attending the games.
Russia continues to face a bloody insurgency in its southern regions, pitting the security services against Islamist militants determined to establish an independent Islamic caliphate.
More than 500 people were killed in fighting across the North Caucasus last year.
In recent weeks, the Russian military has stepped up raids against suspected militants in an effort to secure the area ahead of the games.
While an attack on the Olympic Village itself - like the 1972 massacre in Munich-- is thought unlikely but both Russian and Western intelligence fear that militants could target the city of Sochi or the surrounding area.
Telegraph Media Group Limited