The US Homeland Security Department contacted several airports and airlines on Wednesday over the possibility that terrorists may attempt to smuggle concealed explosives into Russia, broadcaster ABC News reported.

The report cited a senior US official, speaking on a condition of anonymity, as the source of the news.

The official said the US and foreign airlines were warned that the bomb-making material could be assembled in flight or upon arrival at the Olympics. The source did not disclose whether any specific intelligence prompted the warning.

In a statement, the US Homeland Security Department said it "isn't aware of a specific threat to the homeland at this time."

It said the department "regularly shares information with domestic and international partners, including those associated with international events such as the Sochi Olympics."

Russian security forces are on high alert for possible terror threats ahead of the Winter Olympic Games, which officially open on Friday.

Fears of violence were raised in December when suicide bombers killed 34 people in the Russian city of Volgograd, 400 miles (700 km) northeast of Sochi.

Ban speaks out on gay rights

As well as the threat of terrorism, the build-up to the Winter Olympics has been overshadowed by Russia's attitude towards human rights. President Vladimir Putin has been criticized since the passing of an anti-gay propaganda law in June 2013.

Putin has said homosexuals will not be discriminated against during the Olympics, but United Nations Secretary General Ban Ki-moon broached the subject in a speech to the International Olympic Committee on Thursday.

"Many professional athletes, gay and straight, are speaking out against prejudice," he said.

"We must all raise our voices against attacks on lesbians, gays, bisexual, transgender or intersex people. We must oppose the arrest, imprisonment and discriminatory restrictions they face."

Ban's words came the same day British newspaper The Guardian published an open letter penned by more than 200 prominent international authors criticizing Russia's "chokehold" on creativity. Salman Rushdie, Günter Grass and Margaret Atwood were all among the letter's signees

"Russian lawmakers have passed a number of laws that place a chokehold on the right to express oneself freely in Russia," the letter read. "Three of these laws specifically put writers at risk: the so-called gay 'propaganda' and 'blasphemy' laws, prohibiting the 'promotion' of homosexuality and 'religious insult' respectively, and the
recriminalisation of defamation.

"... We therefore urge the Russian authorities to repeal these laws that strangle free speech."

Competition in Sochi began on Thursday, with qualifying underway in men's and women's snowboard slopestyling, women's freestyle moguls skiing and team figure skating events.

ccp, ph/tj (AP, AFP dpa, Reuters)