German Chancellor Angela Merkel said an EU-Turkey deal to halt illegal migration to Europe was bound to run into setbacks and faced big legal challenges but nevertheless had an "irreversible momentum".
Merkel voiced satisfaction but also caution at a news conference after the deal was agreed.
She said: "I have no illusions that what we agreed today will be accompanied by further setbacks. There are big legal challenges that we must now overcome.
"But I think we've reached an agreement that has an irreversible momentum and it was very important for me that we managed to agree all that among the 28 today."
The agreement involves closing the EU border on March 20 and sending back migrants who arrive in Greece after that date to Turkey.
Cameron asks EU leaders to back deployment of more international patrol ships close to Libyan coast in bid to deter migrant smugglers.
David Cameron will tell his European colleagues that the migration route from Libya to southern Italy is about to begin once more.