Lebanese Prime Minister Tammam Salam is pleading for calm as anger swells in his country over the continued capture of soldiers and police by militants in Syria.

In a rare televised address on Lebanese television stations on Sunday, Salam urged his countrymen to trust the government's efforts to negotiate the men's release. It came after photos emerged showing that militants of the Islamic State group had beheaded a second captive soldier on Saturday.

Militants in Syria, including the Islamic State group, are holding around 20 soldiers and police. They were seized after militants briefly overran the Lebanese border town of Arsal in August.

Families of the captive men have blocked Lebanon's highways, burning tires and sending gunmen to the streets. Others have vowed to punish Syrian refugees in Lebanon.