World leaders have condemned Friday's terror attacks in Paris, as cities across the world stepped up security in response to the killings. The UK prime minister, David Cameron, warned the country to be prepared for the possibility that Britons could be among the casualties, while the Queen expressed her shock and sadness at the killings. In a message to her fellow head of state, François Hollande, she said: "Prince Philip and I have been deeply shocked and saddened by the terrible loss of life in Paris. We send our most sincere condolences to you, the families of those who have died and the French people." The US president, Barack Obama, condemned the atrocities, in which at least 127 people died. And on Friday night, the United Nations security Ccouncil called the attacks "barbaric and cowardly". The Russian president, Vladimir Putin, and the Pope were also among the world leaders to express their condolences. They were joined by the London mayor, Boris Johnson, who said he had written to his Parisian counterpart, Anne Hidalgo. Live Paris attacks: Islamic State says killings were response to Syria strikes – live Latest coverage of the attacks across Paris that have left at least 127 people dead Read more In an address at the White House, Obama said: "It's an attack not just on the people of France. But this is an attack on all of humanity and the universal values we share. "We're going to do whatever it takes to work with the French people and with nations around the world to bring these terrorists to justice and to go after any terrorist networks that go after our people." The US secretary of state, John Kerry, called the attacks "heinous, evil" and "vile" and "an assault on our common humanity". Kerry said the American embassy in Paris was trying to confirm the safety of US citizens, adding that Washington was ready to provide whatever support the French government may require. Putin sent his deep condolences to Hollande and the people of France following the "horrible terrorist attacks in Paris", the Kremlin said in a statement. "Russia strongly condemns this inhumane killing and is ready to provide any and all assistance to investigate these terrorist crimes." Pope Francis added his condemnation, telling the Catholic television channel TV2000 the attacks were "not human" and "there is no religious or human justification for it". He said: "I am close to the people of France, to the families of the victims, and I am praying for all of them. I am moved and I am saddened. I do not understand, these things hard to understand." The leaders of other countries that have been targeted by terrorism joined the chorus of condemnation. Jose Manuel Garcia Margallo, the foreign minister of Spain, where 191 people were killed in train bombings in 2004, warned of possible repeat attacks by jihadis. "All of this confirms that we are facing an unprecedented challenge, a hugely cruel challenge", he told public television TVE. India's prime minister, Narendra Modi, whose country was hit by two attacks in 2006 and 2008, said on Twitter the "news from Paris is anguishing & dreadful". The Israeli prime minister, Binyamin Netanyahu, said: "Israel stands shoulder to shoulder with French president Francois Hollande and with the people of France in our common battle against terrorism." The German chancellor, Angela Merkel, said she was "profoundly shocked by the news and images from Paris". Merkel issued a statement saying her thoughts were with the victims, while the Dutch foreign minister, Bert Koenders, said he was "shocked and appalled by new attacks in Paris. Words are not enough." Paris attack witness: 'he was dressed in black, professional, shooting and killing' Read more Canada's new prime minister, Justin Trudeau, said: "Our hearts and thoughts and prayers go out to our French cousins in this dark and terrible time", adding that Canada had offered "all of our help and support to the government of France". In Australia, the prime minister, Malcolm Turnbull, said the attacks appeared "to have all the hallmarks of Daesh, of Isil", but that information was still limited. He pledged Australian solidarity and support for the fight against terrorism. "It is a global struggle for freedom against those who seek to suppress it and seek to assert some form of religious tyranny; a threat in the name of God but [which] is truthfully the work of the devil," he said. The foreign minister, Julie Bishop, said: "This is indeed a black Friday for France and for the world." After waking up to the news from Paris, countries in Asia voiced anger at the attacks and solidarity with the people of France. China's president, Xi Jinping, spoke of "these tragic times for the French people" and condemned "in the strongest ways this barbarous act". Chinese state television quoted Xi as saying Beijing would "battle terrorism along with France and the international community". Iran's president, Hassan Rouhani, postponed his trip to Europe after Friday's terror in Paris. He strongly condemned the deadly attacks, branding them "crimes against humanity" in a message to Hollande, official news agency Irna said. "In the name of the Iranian people, who have themselves been victims of terrorism, I strongly condemn these crimes against humanity and offer my condolences to the grieving French people and government," Rouhani wrote. Singapore raised its alert level, with the foreign minister, Vivian Balakrishnan, describing the French attacks as "a terrible assault on a beautiful city with warm, cheerful, hospitable people". He added: "We must not let the terrorists divide us or destroy our spirit. I know the French spirit will prevail." Jens Stoltenberg, the secretary general of Nato, said he was "deeply shocked" by the Paris attacks. "We stand together with the people of France. Terrorism will never defeat democracy," he said on Twitter. US presidential candidates condemned the attacks and voiced support for the French government and people. Donald Trump said his prayers were with the victims and hostages. Trump's Republican rival, Jeb Bush, said: "America will stand with you against terror."