Hussein Malla, Associated Press
This Thursday, Sept. 4, 2014, a family of a missing Lebanese soldier who was kidnapped by Islamic State militants, sits on the ground as they block a street during a demonstration to demand action to secure the captives' release, in front the Lebanese government building, in downtown Beirut, Lebanon.

BEIRUT — Lebanon's prime minister pleaded for his countrymen to be calm in a rare televised appeal on Sunday evening, as anger swelled over the continued capture of soldiers and police by militants in Syria.

Tammam Salam's call came after photos emerged showing that militants of the Islamic State group had beheaded a second captive Lebanese 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, the most serious spillover yet of Syria's conflict into the neighboring country.

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.

"Resorting to shutting down roads, and stopping movement in the country will not return to us our soldiers. Our battle is with our terrorist enemy, not against each other," Salam said.

The men's continued detention has also inflamed already bad sectarian relations in Lebanon between Sunnis and Shiites. Relations have been steadily worsening, inflamed over the Syrian civil war, as Sunnis generally back rebel groups and Shiites support the government of President Bashar Assad.

The Lebanese Shiite militia Hezbollah has actively fought on the Syrian government side, and some Sunnis blame Hezbollah's intervention in the Syria war for the soldiers being seized. Some Shiites see Sunni extremism as the chief problem.

"But we must know that the discord that the terrorists seek, made easy by the ignorant and the easily misled, provides an entry for the destruction of our national harmony," Salam said.

He warned that militants may kill more captives, but urged his countrymen to trust the government's efforts to negotiate their release.