Russia and Syria are deliberately using migration as an aggressive strategy towards Europe, the senior Nato commander in Europe has said.
US Gen Philip Breedlove said they were "weaponising" migration to destabilise and undermine the continent.
He also suggested that criminals, extremists and fighters were hiding in the flow of migrants.
Migrants are continuing to accumulate in Greece, after Macedonia stopped allowing more than a trickle through.
On Wednesday it allowed around 200 Syrian and Iraqi refugees to cross, with thousands still stuck on the Greek side of the border.
New figures suggest last year's total of one million seaborne migrants arriving in Europe could be matched well before the end of the year.
The International Organization for Migration (IOM) said nearly 129,500 migrants had arrived by sea so far in 2016, plus another 1,545 by land. It said 418 had drowned or were missing.
The crisis has caused tensions to surge, with Greece struggling to cope with the influx and the European Commission criticising Macedonia for using tear gas on a crowd of migrants on Monday morning.
"The scenes we just saw are not our idea of managing the crisis," said EC spokesman Margaritis Schinas.
In the Jungle camp of migrants in Calais, France, the demolition of the southern half of the camp continues - in what the government has termed a humanitarian operation but which critics say will just leave hundreds of desperate migrants without shelter in winter.
A volunteer with an aid organisation in the camp told the BBC that children were in danger in the camp, saying that she had spoken to children who had been raped and who were carrying out sex work.
'On the road'
Gen Breedlove is the head of the US European Command as well as Nato's Supreme Allied Commander Europe.
He told the US Senate Armed Services Committee that the crisis was allowing Russia to use non-military means to create divisions in the Nato alliance and Europe.
Analysis by BBC Europe correspondent Chris Morris
Greece is now the bottleneck in the migration crisis and six years of deep financial crisis have reduced the ability of the Greek state to respond quickly and effectively on its own. So the European Commission is assuming that it will have to help Greece look after a shifting population of about 100,000 refugees and migrants for the foreseeable future.
But that figure only makes sense if considerable progress is also made in reducing the number of arrivals by sea, from Turkey into the Greek islands. The rate of arrivals has fallen in recent days, but the average so far this year is close to 2,000 people per day. That means greater co-operation from Turkey remains essential if the EU is to succeed.
One sign of progress is the announcement that about 300 irregular migrants from North Africa are being returned to Turkey from Greece this week, under a little-used bilateral agreement. But its effect will be limited. The vast majority of recent arrivals are from Syria, Iraq and Afghanistan - three countries in which civil wars continue to rage, and from which asylum applications are routinely accepted.
So the strain on the system in Greece will continue to be substantial. And the fact that huge amounts of European humanitarian aid will be spent here, in the same way that it's spent in conflict zones in the developing world, is a striking symbol of the depth of Europe's migration crisis.
Russia and Syria's leader Bashar al-Assad, Gen Breedlove said, were "deliberately weaponising migration in an attempt to overwhelm European structures and break European resolve".
He cited the use of barrel bombs - unguided weapons - against civilians in Syria. The only purpose of these indiscriminate attacks was to terrorise Syrian citizens and "get them on the road" to create problems for other countries, Gen Breedlove said.
Gen Breedlove added that violent extremists, fighters and criminals - including elements from the extremist Islamic State group - were in the mix of migrants.
He said he had requested that more US forces be permanently based in Europe. Their numbers have dropped from a Cold War high of half a million to about 62,000.
The European Commission has now adopted plans to disburse €700m (£543m; $760m) of emergency humanitarian funding between 2016-18 to help tackle the crisis, says humanitarian aid commissioner Christos Stylianides.
Under the plan, which still needs approval by the European Council and Parliament:
- EU aid agencies would for the first time work directly with the UN and other groups inside Europe, using monies usually allocated to emergencies outside its borders
- €300m would be spent this this year, and €200m each the following two years, to help any EU state deal with the migration crisis
Greece has asked the European Commission for nearly €500m in assistance to help care for 100,000 asylum seekers.
"We cannot bear the strain of all the refugees coming here," government spokeswoman Olga Gerovassili was quoted as saying.
Despite commitments to relocate 66,400 refugees from Greece, EU member states have so far pledged just 1,539 spaces and only 325 people have actually been relocated, Reuters quoted a spokesman for the UN refugee agency as saying.
Meanwhile Turkey has expressed frustration at the lack of a common position from the EU on the crisis.
"If the burden is going to be lifted from Turkey, we should be hearing something about... giving the Syrians a legal possibility so they can go to EU countries without having to brave the waters... the EU has to get its act in order," Selim Yenel, the Turkish ambassador to the EU, told the BBC's HardTalk programme.