BRUSSELS/VIENNA (Reuters) - Austria threatened to reimpose controls on its Hungarian border and Britain considered adding security around the French port of Calais on Wednesday as divided EU leaders prepared to debate how to stem a flood of desperate migrants.
On the eve of a Brussels summit looking for ways to spread the load of receiving hundreds of thousands fleeing poverty and war in Africa and the Middle East, the frontier rows across the 28-nation bloc were a reminder that the stakes are high.
"Migration ... is a transformative challenge," one senior EU diplomat said, noting the strain it was putting on unity among neighbors where years of economic drift have bolstered anti-immigration parties, even as turmoil to the south has driven growing numbers to risk perilous crossings of the Mediterranean.
Austria's warning about reinstating checks on its passport-free border with Hungary follows a refusal by Budapest to take in asylum seekers sent back to it by other member states under EU rules. Hungary says it no longer has the means to deal with a sharp increase in people streaming over its Balkan border - many from the Middle East, including via crisis-wracked Greece.
British Prime Minister David Cameron, who will also use the summit to launch demands to limit free access for other EU citizens ahead of referendum on EU membership, played down any tension with France over thousands of migrants massed around Calais hoping to reach Britain by ferry or the Channel Tunnel.
But he said his ministers were looking at ways to add border guards and dogs around the port of Calais and the entrance to the rail tunnel after dramatic scenes on Tuesday when disruption caused by a ferry strike gave an opportunity to large crowds to try to stow away on trucks waiting in line.
"We should work with the French closely," Cameron told parliament after criticism from the mayor of Calais.
"There is no point either side trying to point the finger of blame," Cameron said. "This is a strong partnership that we have in place and we should keep it that way."
Many governments from across the continent say the system for tackling migration is broken. But Hungarian Prime Minister Viktor Orban's decision to halt transfers of asylum seekers is the most radical step taken by any European leader so far.
Orban has a history of tangling with Brussels, notably over civil rights. European Commission President Jean-Claude Juncker has taken to greeting him, with barbed humor, as "the dictator". Orban, meanwhile, is under political pressure at home from an anti-immigrant far-right opposition party.
Under EU rules, migrants must apply for asylum in the first member state they enter. If they move on to another EU country, they can be sent back to the country where they entered.
Hungary said it was suspending such transfers because it was overwhelmed by migrants, after 61,000 crossed into the country from outside the EU since the start of the year. That angered Austria, where many migrants head after passing through Hungary.
"Hungary’s decision is completely unacceptable for us," Austrian Interior Minister Johanna Mikl-Leitner said. "We do not rule out border controls as a last resort."
Hungary stood its ground. Foreign Minister Peter Szijjarto said Austria and other EU states were planning to send back to Hungary illegal immigrants who should instead be sent to Greece.
Italy, Greece and Malta, where migrants make landfall after crossing the Mediterranean, have borne the brunt of the recent influx. But there is also an overland route and Hungarians complain Greece, embroiled in economic crisis and arguments with its creditors, is not doing enough to stop migrants passing on.
Orban's government earlier this month ordered the construction of a fence along Hungary's border with Serbia to keep people from crossing illegally.
EU leaders were pushed to act after some 800 people drowned on one boat of migrants heading for Italy in April.
But they are divided on a plan from the EU executive to oblige them to take a share of 24,000 Syrians and Eritreans now in Italy and 16,000 from Greece. Diplomats say they may compromise by accepting such numbers on a voluntary basis.
The summit, according to draft conclusions seen by Reuters, is likely to give states -- including long-standing members Britain, Ireland and Denmark which have opt-outs on such issues -- until the end of July to work out how to distribute the 40,000 people over the coming two years. A further 20,000 people already classed as refugees will be taken in by EU states.
Poorer, eastern states in particular complain they do not have the capacity to take the numbers originally proposed.
Longer-term responses - such as boosting aid to countries where emigrants leave from, discouraging economic migrants while they are still in Africa and accelerating the return of failed asylum seekers - are also on the summit agenda.
Some states are pushing hard to make a link between African countries doing more to control their own borders to stem emigration and their access to more EU aid.
On Monday, EU foreign ministers agreed a naval operation to combat people-traffickers who have carried thousands of migrants on perilous journeys across the Mediterranean.
Ships patrolling the Mediterranean have plucked more than 3,700 migrants from overcrowded and unsafe boats on Monday and Tuesday, Italy's coast guard said. Italy estimates 60,000 people have made it across so far this year while the U.N. says almost 2,000 have died in the attempt.
Additional reporting by Kylie McLellan in London, Krisztina Than in Budapest and Paul Taylor in Brussels; Writing by Alastair Macdonald; Editing by Andrew Hay