An Israeli air strike Saturday flattened a 13-floor building housing Qatar-based Al Jazeera television and the US news agency the Associated Press in the Gaza Strip.

Israel "destroyed Jala Tower in the Gaza Strip, which contains the Al Jazeera and other international press offices," Al Jazeera said in a tweet, with an AP journalist saying the army had warned the tower's owner ahead of the strike.

AFP journalists, whose Gaza offices are in a different place, watched the building collapse to the ground after the Israeli air strike, sending up a huge mushroom cloud of dust and debris.

Israeli defence officials said the building housed not only news bureaus but offices of Hamas militants.

But Walid al-Omari, Al Jazeera Jerusalem bureau chief, was incensed and vowed the network will not be silenced.

-It is clear that those who are waging this war do not only want to spread destruction and death in Gaza, but also to silence media that are witnessing, documenting and reporting the truth- he said.

AP President and CEO Gary Pruitt said he was "shocked and horrified" by the attack, adding about a dozen AP journalists and freelancers had been in the office, but were able to flee.

-This is an incredibly disturbing development. We narrowly avoided a terrible loss of life- Pruitt said.

The White House, which has kept a relatively low profile on the Israeli-Palestinian flare-up, said it has cautioned Israel about the importance of protecting independent media.

-We have communicated directly to the Israelis that ensuring the safety and security of journalists and independent media is a paramount responsibility- White House press secretary Jen Psaki tweeted.

Jawad Mehdi, the owner of the Jala Tower, said an Israeli intelligence officer warned him he had just one hour to ensure that the building was evacuated.

In a phone call with the officer, he begged for an extra 10 minutes to allow journalists to retrieve their equipment before leaving.

-Give us ten extra minutes- he urged live on television, but the officer on the other end of the line refused.

AP journalist Fares Akram wrote on Twitter shortly before the strike that he and colleagues "ran down the stairs from the 11th floor" to get away safely.

Mostefa Souag, acting director of the Al Jazeera Media Network, denounced the strike and called for international condemnation.

-The aim of this heinous crime is to silence the media and to hide the untold carnage and suffering of the people of Gaza- he said in a statement.

'Everything became rubble'

After the air raid, Al Jazeera correspondent in Gaza, Safwat al-Kahlout, pledged in a phone call with the Qatari broadcaster to continue coverage "despite the destruction".

-Suddenly everything became rubble- he said. -We will be on air again with new equipment.-

His colleague Harry Fawcett described the strike as "extraordinary".

-This is a very personal moment for all of us. The idea that the place is not there anymore is extraordinary to contemplate- he told the broadcaster.

Israel said its "fighter jets attacked a high-rise building which hosted military assets belonging to the military intelligence of the Hamas terror organisation".

-The building also hosted offices of civilian media outlets, which the Hamas terror group hides behind and uses as human shields- it said.

Israeli air and artillery strikes on Gaza since Monday have killed 139 people including 39 children, and wounded 1,000 more, health officials in the besieged coastal enclave say.

Palestinian armed groups have fired 2,300 rockets at Israel, killing 10 people, including a child and a soldier, and wounding over 560 Israelis.

More than half of the missiles fired from the Hamas-controlled Gaza Strip have been intercepted by Israel's air defence system.