Relatives of Lebanese soldiers held hostage by Islamic militants have burned tires and closed roads in downtown Beirut Monday in an escalation of their ongoing sit-in to secure their release.

Monday's protests began after the militants threatened to start killing the hostages within hours unless the government revokes sentences handed down to Islamist prisoners.

The militants, including the al-Qaida-linked Nusra Front and the extremist Islamic State group, have been holding some 20 Lebanese soldiers and policemen hostage since August, when they briefly overran a Lebanese border town.

Since then, at least three have been killed by their captors. Families of remaining soldiers have pitched tents in downtown Beirut opposite the prime minister's office in an effort to pressure the government into mediating with the captors.