Fears of a terrorist attack in Europe were heightened today after travel alerts were issued by both Washington and London, with the US state department warning that public transport systems and tourist attractions could be targeted, and Britain increasing the threat level for travel to Germany and France.

The US alert made no mention of a specific European country, but pointed the finger at al-Qaida and its affiliates and suggested both official and private targets could be in terrorists' sights.

The move follows speculation that al-Qaida was planning a "commando-style" attack similar to the 2008 Mumbai massacre, in which 166 people were killed.

The Foreign Office listed both France and Germany as facing a high threat of terrorism, raising its previous classification of a general threat. It said attacks could target places frequented by tourists.

However, well-placed British sources described intelligence about potential commando-style attacks as "ill-defined".

In a separate move, the home secretary, Theresa May, said the threat level for Britain had not changed, remaining severe, meaning an attack is "highly likely". She said the UK and US were in agreement as to what was going on. "I would urge the public to report suspicious activity to the police in support of the efforts of our security services to discover, track and disrupt terrorist activity," she said.

The revised warnings come amid signs of growing jitters across Europe about planned attacks. In Paris, the Eiffel Tower has been closed twice in two weeks, the last time on Tuesday when a bomb warning was called in from a telephone booth. The building was searched, but nothing was found. Sweden also increased its threat level on Friday.

But Germany said today it was not changing its risk assessment as there are "still no concrete indications of imminent attacks" – despite a CNN report that thousands of US troops stationed at the Ramstein Air Base were ordered to stay in their homes on Friday night due to rising concerns about a possible attack.

European officials said there was no intelligence suggesting an attack was imminent, and warned against linking recent US attacks on suspected militant bases in Pakistan with suggestions of "commando-style" attacks in Europe. The attacks were directed against militants in Waziristan, the Pakistani tribal area that borders Afghanistan. US officials have been reported as saying that Osama bin Laden and his inner circle may have been involved in co-ordinating plans for a number of attacks on European cities, timed to occur simultaneously.

An intelligence official in Pakistan has been reported as saying that two British brothers, one of whom was killed in drone attacks, along with eight Germans, were involved in the planning.

The New York Times said that information about the plot had been acquired by intelligence agencies in part by interrogating Ahmed Sidiqi, 36, a German of Afghan origin who had travelled from Hamburg to Waziristan to receive arms and explosives training at a militant camp. Sidiqi was reported to have been captured in Afghanistan in July.

Though couched in cautious terms, the US travel alert is unusually specific in its reference to danger spots. It mentions public transport and tourist venues, reminding US travellers that terrorists have targeted "subway and rail systems, as well as aviation and maritime services".

European capitals are understood to have made no efforts to dissuade the US from issuing the travel alert, suggesting broad concurrence.

In recent years anti-terrorist operations across Europe, backed by intelligence sharing with the US, have been successful in preventing another atrocity. The last large-scale attack in Europe was the London suicide bombings of July 2005 in which 52 people were killed.