The Prime Minister and the Foreign Secretary have led tributes to two Western journalists, including Marie Colvin of the Sunday Times, who have been killed in an attack in the Syrian city of Homs.
Award-winning French photographer Remi Ochlik , 28, also died in the artillery attack, which is said to have targeted an opposition safehouse.
They were among 26 people killed on Wednesday as Syrian government forces pounded the rebel city for a 19th day, activists said.
An estimated 5,400 civilians have died in the country since protests against the regime of President Bashar al Assad erupted last March, according to the UN.
Paying tribute to Ms Colvin, 55, David Cameron told the House of Commons: "This is a desperately sad reminder of the risks that journalists take to inform the world of what is happening and the dreadful events in Syria, and our thoughts should be with her family and her friends."
William Hague added: "Marie and Remi died bringing us the truth about what is happening to the people of Homs.
"Governments around the world have the responsibility to act upon that truth - and to redouble our efforts to stop the Assad regime's despicable campaign of terror in Syria."
The White House described the deaths as "tragic" and offered its condolences to the journalists' families.
The Foreign Office earlier summoned Syria's ambassador to London, Sami Khiyami, for a meeting in which demands were made for the Syrian authorities to immediately repatriate the journalists' bodies and provide medical treatment for photographer, Paul Conroy, who was with Ms Colvin.
Ms Colvin, an American, had been a war reporter for the Sunday Times for 20 years. Her career took her to some of the world's most dangerous conflict zones, and she continued working even after losing an eye to a shrapnel wound in Sri Lanka in 2001.
In broadcasts on Tuesday night, she had charged the Syrian army with a "complete and utter lie that they are only targeting terrorists" and described the scenes in Homs and attacks on "cold, starving civilians" as "absolutely sickening".
Syrian authorities said they did not know the dead journalists were in Syria. Ms Colvin herself had written in her final article for the Sunday Times that she had entered Syria on a "smugglers' route".
Sky News' chief correspondent, Stuart Ramsay, had dinner with Ms Colvin and Mr Conroy the night before they crossed into Syria.
Ramsay said: "She was determined to get the story out. What she said carried a lot of weight with people around the world.
"On this occasion, she was hit by mortar fire in a stairwell as they decided to leave to find a safer location. It's horrendous there. A daily grind of missile after missile after missile."
The editor of the Sunday Times, John Witherow, described Ms Colvin as "an extraordinary figure in the life of The Sunday Times, driven by a passion to cover wars in the belief that what she did mattered".
"She believed profoundly that reporting could curtail the excesses of brutal regimes and make the international community take notice," he added.
"Above all, as we saw in her powerful report last weekend, her thoughts were with the victims of violence.
"She was a woman with a tremendous joie de vivre, full of humour and mischief and surrounded by a large circle of friends, all of whom feared the consequences of her bravery."
Rupert Murdoch, head of News Corporation, whose UK subsidiary News International publishes the Sunday Times, told staff in a memo: "We are doing all we can in the face of shelling and sniper fire to get (Paul Conroy) to safety and to recover Marie's body."
Mr Ochlik is the second French journalist to be killed in the unrest in Syria after cameraman Gilles Jacquier died last month in a mortar attack during an authorised press trip, also in Homs.
Upon hearing news of his death, the French government called on Syria to make an immediate stop to the bombardments in Homs, and to allow safe passage for humanitarian aid to reach affected areas .
French foreign minister Alain Juppe, who earlier summoned Syria's envoy to France to demand an end to the violence, said France held the Syrian authorities responsible for the journalists' killing.
Activists said a number of other local journalists and activists were killed and injured in the attack.
They died in the Babr Amr district of Homs, an area which has suffered the brunt of a government assault in recent weeks that activists say has left hundreds dead in the city.
The Syrian National Council (SNC) blamed the journalists' deaths on government forces.
"I see no reason why opposition members would shoot at journalists," said SNC spokeswoman Basma Qadmani. "It is, therefore, most probably related to the regime."
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