UK Prime Minister David Cameron has announced that he is drawing up plans to help France tackle a spike in attempts by migrants to enter Britain illegally via the Channel Tunnel.

"The situation is not acceptable. Starting with helping the French on their side of the border, we're going to put in more fencing, more resources, more sniffer dog teams, more assistance in any way we can," Cameron said on Friday following a meeting with Britain's emergency response committee 'Cobra'.

Cameron is under pressure to deter the migrants, many of whom have travelled from Africa and the Middle East, after disruption to cross-Channel passenger and freight traffic.

"This is going to be a difficult issue right across the summer."

Britain has pledged $34m so far towards improving security at the French end of the tunnel that connects both the countries.

French police have in recent days prevented thousands of desperate attempts to scale fences around the tunnel terminal in Coquelles near Calais, although many migrants have successfully made it across.

France sent 120 additional officers to the site this week.

Cameron was expected to speak to French President Francois Hollande later on Friday to discuss the issue.

Political headache

Re-elected in May, Cameron has promised to cut net annual migration to Britain to the tens of thousands, a pledge he failed to keep during his 2010-15 term in office when it hit a near record high of over 300,000 people.

The issue is a sensitive one as it plays into Britain's debate about Europe ahead of an EU membership referendum Cameron has promised by the end of 2017.

Migrants have long gathered in Calais to try to get into Britain. But Eurotunnel, the firm that runs freight and passenger shuttles via the Channel Tunnel, said the numbers have swelled to around 5,000 people from about 600 and that it is struggling to cope.

It said migrants have also become better organised, mounting nightly attempts in large groups to storm the facilities.

Eurotunnel has sometimes been forced to suspend its services, causing disruption at what is one of the busiest times of the year for British holidaymakers.

The situation has caught the imagination of Britain's tabloid newspapers, becoming a political headache for Cameron.

He is under pressure to get tough on the migrants from many lawmakers in his ruling Conservatives. But he also has to contend with political rivals unhappy with the debate's tone.

The opposition Labour Party has criticised Cameron for referring to the migrants as a "swarm", saying the term was dehumanising and stirred public hostility against people sometimes fleeing poverty and conflict.