The migrant crisis could shift from Calais to other ports, Theresa May has warned.
Talks between the British Government and officials in the Netherlands and Belgium have started amid concern people smugglers may target different routes. The development emerged as the Home Secretary signed a fresh agreement with France to tackle the situation in Calais, where up to 5,000 migrants are massed.
VIsiting the port town in northern France for the first time since the crisis escalated, Mrs May inspected a ring of steel surrounding the Eurotunnel terminal at Coquelles.
She insisted the new measures - part of a £7 million investment by the UK - are taking effect but admitted there are concerns the defences could force migrants to move to other European ports.
Immigration Minister James Brokenshire has spoken to officials in the Belgium and the Netherlands and is expected to travel for further discussions.
Mrs May also said other ports in France such as Dunkirk are being scrutinised. She said: "We are also looking at the security of other ports. We are very well aware of the possibility of displacement."
Zeebrugge in Belgium and the Hook of Holland are seen as potentially vulnerable.
Mrs May claimed measures announced in recent weeks have had an impact. She said: "We have already taken a number of steps that have started to improve the situation here in terms of numbers of people trying to access the tunnel and get through to the UK. But the work must continue."
Britain has committed more than £20 million towards efforts to tackle the wider crisis in the last year.
At Coquelles a 2km stretch of four metre high barbed wire fencing has been erected at the freight and vehicle terminal, which has been a hotspot for migrant incursions. The barrier will be extended by a further 1.5km on each side to reach the mouth of the tunnel.
The number of attempts to board lorries or trains has fallen from 2,000 a night at its peak to around 200.
Steps in the accord signed today by Mrs May and her French counterpart Bernard Cazeneuve include:
:: A new command and control centre in Calais to target organised immigration crime. Details are yet to be thrashed out but its work will come under the command of French police and will involve UK law enforcement personnel.
:: Britain will spend £7 million to help manage migrants in Calais and set up facilities to process asylum seekers.
:: Visits by Border Force officials to migrant camps to warn occupants that Britain's streets are "not paved with gold".
:: UK support for flights to return migrants to home countries.
Despite the latest package of measures, Mrs May denied that the UK was contributing resources to ease a French problem.
She said: "What is important is that we are both working together. I think it is entirely right that the UK Government is playing its part."
The crisis in Calais is part of a wider migrant surge into Europe from north Africa and the Middle East.
Those trying to make the journey include some refugees and "many, many" economic migrants, Mrs May said.
She repeated her call for people trafficking gangs to be "relentlessly" pursued.
Philip Duffy, chief operating officer of the UK Border Force, said organised gangs attempt to smuggle migrants into Britain to commit crime.