The United States on Friday warned Americans to be wary of terror strikes all over the world, following an attack on French satirical magazine Charlie Hebdo in Paris on Wednesday which left 12 people dead.

Another four were killed in separate sieges at a Jewish supermarket and a print shop on Friday.

Four hostages died in the Paris supermarket siege

"Recent terror attacks, whether by those affiliated by terrorist entities, copycats, or individual perpetrators, serve as a reminder that US citizens need to maintain a high level of vigilance and take appropriate steps to increase their security awareness," the US State Department wrote in its travel warning.

The advisory also said that US airstrikes on "Islamic State" militants in Iraq and Syria meant that US nationals and nationals of US coalition countries were particularly prone to reprisals by jihadist militants.

It cited threats issued by extremists calling on supporters "to attack foreigners wherever they are."

Security tightened in Britain and France

In a TV interview on Friday evening, French Prime Minister Manuel Valls said authorities were bracing for possible further attacks.

"We are facing a major challenge" and "very determined individuals," Valls said.

Britain's government also said on Friday that it had beefed up security at its seaports and transport links to France, particularly in Calais and at the Eurostar terminal at Gare du Nord in Paris. A spokeswoman for British Prime Minister David Cameron said the extra security was "on a precautionary basis."

On Wednesday, three men entered the Charlie Hebdo offices in central Paris, killing 12 people. French police subsequently launched a massive hunt for those responsible.

On Friday, the suspects surfaced and took a print shop and a kosher supermarket hostage for several hours before police raided the premises, killing the men.

Following Friday's events, the death toll has now climbed to 17. A policewoman was killed on Thursday in a related incident.

Solidarity against extremists

US President Barack Obama has meanwhile expressed his country's support: "I want the people of France to know that the United States stands with you today, stands with you tomorrow," he said.

Israel's President Benjamin Netanyahu offered his condolences, asking for France to maintain tight security over Jewish sites. Israel's sworn enemies Iran and Hezbollah also condemned the attacks.

"Through their shameful, heinous, inhumane and cruel words and acts, [these groups] have offended the prophet, religion, the holy book and the Muslim people more than any other enemy," said Sheikh Hassan Nasrallah, leader of the Lebanese Hezbollah group.

North Korean Foreign Minister Ri Su Yong also sent a message of condolence, saying his country was opposed to "all sorts of terrorism."

World leaders will show their support on Sunday in Paris

The French government is planning a march of national unity on Sunday in Paris. Hundreds of thousands are expected to attend, including German Chancellor Angela Merkel, UK Prime Minister David Cameron and Italian Prime Minister Matteo Renzi.

Cameron said the rally would celebrate "the values behind Charlie Hebdo."

The leaders of Belgium, Portugal, Poland, Sweden, Denmark, Norway and Ukraine also planned to attend, along with many high-profile French politicians.

mg/cmk (dpa, AP, AFP, Reuters)