Media playback is unsupported on your device Media caption The BBC's Daniel Sandford in Sochi says there is a strong security presence at the airport

The US has warned airlines with direct flights to Russia that explosives hidden in toothpaste tubes could be smuggled onto planes.

The Department of Homeland Security's warning comes ahead of the Winter Olympics' opening ceremony in the Russian Black Sea resort of Sochi.

It said it was not aware of any specific danger to the US at this time.

Russia banned all liquids, pastes and gels from its airports and internal flights last month.

The move was part of a significant tightening of security in the country ahead of the Olympic Games, the BBC's Daniel Sandford reports from Sochi.

The Russian authorities have not yet commented on the US warning, but Deputy Prime Minister Dmitry Kozak said the level of security at Sochi "is similar to that of any other safe city in the world".

Although the Games do not officially start until Friday, some qualifying events are now being held.

'Abundance of caution'

Unnamed US security officials were quoted as saying there were fears toothpaste tubes could be used to smuggle explosives which could then be used to assemble a bomb either in flight or upon arrival at the Olympics.

The Department of Homeland Security said in a statement that "out of an abundance of caution" it "regularly shares relevant information with domestic and international partners".

US airline security alerts 2012 - US says it foiled an al-Qaeda plot to attack a US-bound plane using an updated "underwear bomb"

2009 - Nigerian Umar Farouk Abdulmutallab attempts to detonate makeshift bomb smuggled in his underwear on board a Northwest Airlines flight to Detroit on Christmas Day

2006 - Three Britons arrested as they plotted to blow up transatlantic planes with bombs disguised as drinks

2001 - Briton Richard Reid attempts to blow up American Airlines flight from Paris to Miami with explosives hidden in his shoes

"While we are not aware of a specific threat to the homeland at this time, this routine communication is an important part of our commitment to making sure we meet that priority," it added.

The White House National Security Council said the latest threat had not altered existing travel guidelines for Sochi.

"If we should receive information in the coming days and weeks that changes our assessment of whether people should travel to Sochi, we will make that information public," spokeswoman Laura Magnuson told US media.

Air travellers are restricted to carrying liquids - which include pastes, creams, gels and drinks - in containers no larger than 100ml (3.4 oz).

Sweeping changes to airport security regulations were brought in after three Britons were arrested for plotting to bring down transatlantic airliners with home-made liquid explosives disguised as soft drinks in August 2006.

'Olympic Truce'

Fears about security for the Winter Olympics in Russia were heightened following two suicide attacks in Volgograd in December, as well as numerous threats from Islamist militants in the Caucasus region.

The US has also placed two warships in the Black Sea ready to offer help in a case of a security emergency during the Games.

Security has not been the only controversy surrounding the build up to the Games.

Russia's controversial laws on homosexuality, passed last year, have provoked international criticism.

UN Secretary-General Ban Ki-moon referred to the issue in a keynote speech he gave at a session of the International Olympic Committee in Sochi on Thursday.

"Sport has an amazing power to spontaneously unite people," he said. "We must raise our voices against attacks on lesbian, gay, bisexual, transgender or intersex people.

"We must oppose the arrests, imprisonments and discriminatory restrictions they face."

Mr Ban also renewed his call for all warring parties around the world to observe the 'Olympic Truce' and "lay down their weapons during the Games - and to lift their sights to the promise of peace".

The Russian foreign ministry made a similar appeal, saying its call "is primarily addressed to the sides of the blood-letting in Syria".