German ministers questioned major internet companies on Friday about U.S. tracking of web activity, days before a visit to Berlin by President Barack Obama.

Justice Minister Sabine Leutheusser-Schnarrenberger said European-based company representatives of Microsoft and Google didn't have information on the tracking program and open questions remain about the broader issue of intelligence access to user data.

Facebook sent a reply to a series of questions and Apple didn't participate in the meeting.

German ministers already are pressing Washington for information following public disclosures by National Security Agency contractor Edward Snowden of two NSA programs, which collected millions of telephone records and track foreign Internet activity on U.S. networks.

Chancellor Angela Merkel plans to raise the issue with Obama when he visits Berlin next week.

The meeting was called by the justice minister and Vice Chancellor Philipp Roesler, both members of the Free Democratic Party, the junior partner in Merkel's governing coalition. Civil rights including data protection traditionally have been a key issue for the junior party.

In a statement following the meeting, Google said it assured the ministers that it provides user data to governments "only in accordance with the law."

"Our legal team reviews each and every request, and frequently pushes back when requests are overly broad or don't follow the correct process," spokesman Kay Overbeck said in an emailed statement. "And we refuse to participate in any program that requires us to provide governments with access to our systems or to install their equipment on our networks."

Earlier, the British government issued an alert to airlines around the world, urging them not to allow former NSA leaker Snowden to board flights to the United Kingdom.

The alert, dated Monday on a Home Office letterhead, said carriers should deny Snowden boarding because "the individual is highly likely to be refused entry to the UK."

The Associated Press saw a photograph of the document taken Friday at a Thai airport. A British diplomat confirmed that the document was genuine and was sent out to airlines around the world. A Thai airline also confirmed the alert had been issued.

Detrimental to public good

The official said such alerts are issued to carriers that fly into the U.K and any carrier that brings Snowden will be liable to be fined 2,000 British pounds ($3,193 Cdn). He said Snowden would likely have been deemed by the Home Office to be detrimental to the "public good."

The official spoke on condition of anonymity because he was not authorized to discuss the matter publicly.

Snowden, 29, revealed himself Sunday as the source of top-secret documents about U.S. National Security Agency surveillance programs that were reported earlier by the Guardian and Washington Post newspapers. He is believed to be in Hong Kong.

Snowden, an American citizen, has yet to be charged with any crime and no warrants have been issued for his arrest.

The alert was issued by the Risk and Liaison Overseas Network, part of the U.K. Border Agency that has staff in several countries identified as major transit points for inadequately documented passengers.

'Carriers should deny boarding'

The document titled "RALON Carrier Alert 15/13" had a photograph of Snowden and gave his date of birth and passport number. It said: "If this individual attempts to travel the UK: Carriers should deny boarding." It warned that carriers may "be liable to costs relating to the individual's detention and removal" should they allow him to travel.

'Carrier alerts' are issued when the U.K. government wants to deny entry to people who don't normally need visas to enter the country, or already have visas but something has happened since they were issued, said the diplomat. Sometimes convicted sex offenders are denied entry into the U.K. in this way.

A Bangkok Airways officer said the airline was notified on Thursday about the alert by the Airports of Thailand, Pcl., which operates national airports throughout the country. She said the notice was not intended to be seen by the public.

The officer spoke on the condition of anonymity because she was not authorized to give the information to the media.

It was not clear if other countries have posted similar documents.

Britain previously found itself wrapped up in a secret documents leak scandal when Wikileaks founder Julian Assange was granted political asylum last year at the Ecuadorian Embassy in London. He is facing extradition to Sweden where he is wanted on accusations of sex crimes, and has expressed fears that if returned to Sweden he could also face extradition to the U.S.