The US State Department has issued a warning to Americans travelling in Europe over potential terror attacks.

It said in a statement: "Current information suggests that al-Qaeda and affiliated organizations continue to plan terrorist attacks. European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions.

"Terrorists may elect to use a variety of means and weapons and target both official and private interests. US citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure.

"Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services. US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling."

Home Secretary Theresa May backed the warning, saying the message was "consistent" with the UK's view of the current threat level. She also urged the British public to report any suspicious activity as police and security services try to disrupt terrorist activity.

Mrs May added: "As we have consistently made clear, we face a real and serious threat from terrorism. Our threat level remains at severe - meaning that an attack is highly likely. We work closely with our international partners in countering terrorism and the US advice is consistent with our assessment."