ANKARA (AFP) - Britain and Turkey are working "as closely as possible" to stop foreign fighters joining Islamic State in Iraq and Syria (ISIS) insurgents, British Prime Minister David Cameron said on Tuesday.

"We are fighting a common enemy, extremist terrorism," Cameron told a joint news conference in Ankara with his Turkish counterpart Ahmet Davutoglu.

Britain has said it is facing the biggest terrorism threat in its history, in part because of the fear that Britons returning from fighting in Syria and Iraq could launch attacks on home soil.

More than 500 Britons are believed to have crossed into Iraq and Syria to fight with IS militants.

Around half of those are thought to have returned to Britain.

Many extremists are believed to have used Turkey - which shares almost 1,300km of borders with Syria and Iraq - as a transit point to travel to the region.

In Ankara, Cameron said the highest-level intelligence sharing between the two countries could help stop the flow of Islamic extremists in and out of Syria.

"This is all about making sure people are safer in Turkey, and making sure people are safer back home in the United Kingdom," he said.

He said the talks also focused on a "long-term strategy to defeat ISIL and to restore stability in this part of the world," referring to an alternative name for the ISIS group.

"What we need in Iraq is what we need in Syria. We need to see new government that represents all of the people," Cameron said.

Faced with the threat of fighters returning home, his government is planning to toughen its laws to stop potential militants from leaving home.

Those include measures to seize passports from British extremists and stop them returning from fighting overseas, and proposing landing bans on airlines that fail to comply with London's no-fly lists.