Islamist militants holding 19 Lebanese soldiers seized during an incursion into Lebanon said on Friday they had demanded the release of Islamist prisoners in exchange for letting their captives go.

Two commanders close to the group holding the soldiers, speaking by telephone, said the demands had been sent to the Lebanese government and army, which retook the border town of Arsal on Thursday after five days of fighting. It raises the prospect of a potentially lengthy hostage crisis for the Lebanese authorities.

A security source said the militants had put forward the names of around 20 Islamist detainees they wanted released. But a Lebanese government source said the government had not yet received any demand from the gunmen.

One of the militants, speaking to Reuters, said: "It is simple: their soldiers for the Islamic hostages."

The incursion into Arsal by Islamist militants including fighters from the Islamic State - the same group that has seized territory in Iraq and Syria - marked the most serious spillover yet of the three-year-old Syrian civil war into Lebanon.

The militants withdrew from Arsal into the mountainous border region after a battle in which dozens of people were killed, including 17 soldiers.

The militants said their list of Islamist prisoners they want released included Emad Gomaa, whose arrest last Saturday sparked the incursion into Arsal. The army has said the attack by the Islamists had been long-planned.

Gomaa had been a commander in the Nusra Front, al Qaeda's affiliate in the Syrian civil war, but he switched affiliation to the Islamic State in the weeks before his arrest.

The militants are also seeking the release of other Islamists jailed since a 2007 insurrection by an al Qaeda-inspired group at a Palestinian refugee camp in north Lebanon.
Last month, the Nusra Front issued a statement promising to secure the release of Islamist detainees being held at Roumieh prison, northeast of Beirut.

"This has taken a long time and the government does not seem to be willing to listen or understand. Maybe this time they will," said the second militant source. "We have nothing to lose now, we have lost people and our people are refugees again so there is nothing more to lose."

An army patrol entered Arsal town on Friday for the first time since the militants pulled out, a security source said.