A "devastating milestone" has been reached with one million Syrian refugees now in Lebanon which has had a "staggering" impact on the country, the United Nations said.
UN High Commissioner for Refugees Antonio Guterres said: "The influx of a million refugees would be massive in any country. For Lebanon, a small nation beset by internal difficulties, the impact is staggering.
"The Lebanese people have shown striking generosity, but are struggling to cope.
"Lebanon hosts the highest concentration of refugees in recent history. We cannot let it shoulder this burden alone."
Refugees from Syria now equal one quarter of Lebanon's population, according to UN figures.
A suicide car bomb has exploded at a Lebanese army checkpoint in the border town of Arsal, causing casualties, Lebanese security sources have said.
"There are dead and wounded among the soldiers," a security source said.
The town of Arsal is home to thousands of Syrian refugees and also Syrian rebels and their Lebanese allies who have fled an army advance on the Syrian side of the border.
At least three people have died and five have been wounded after a suicide bombing in a Lebanese town near the border with Syria, according to Reuters.
The blast happened in the small town of Nabi Osmane, a stronghold of the militant Hezbollah group, which has been fighting alongside forces loyal to Syrian President Bashar al-Assad.
Syrian soldiers entered eastern districts of the town of Yabroud and advanced towards the main street of the last major rebel bastion near the Lebanese border north of Damascus, Lebanese media reported.
According to Al Mayadeen television, capturing Yabroud would help President Bashar al-Assad secure the land route linking his Mediterranean coastal stronghold with the capital Damascus, and choke off a cross-border rebel supply line from Lebanon.
Mass street protests against Assad broke out three years ago and escalated into civil war after a violent security crackdown on demonstrators.
Israeli warplanes struck a target in eastern Lebanon near the border with Syria on Monday, security sources told Reuters.
It was not immediately known what the target was or the exact location of the air strike, which occurred in a mountainous area near the border.
The Israeli army declined to comment but an Israeli security source confirmed that there has been "unusually intense air force activity in the north", referring to Lebanon.
Israeli airstrikes, believed to target Hezbollah posts or shipments, have been reported at the Lebanese-Syria border.
According to local reports, the two strikes seem to be in Syria's territory.
Two Lebanese soldiers died after a suicide car bomb exploded at an army checkpoint in northeastern Lebanon near the Syrian border, security sources told Reuters.
The attack at the entrance to the city of Hermel, an area where the Shi'ite political and militant movement Hezbollah has a strong presence, also injured several people.
A car bomb has exploded at a Lebanese army checkpoint in the Hermal area of northeastern Lebanon, according to Reuters.
The number of casualties is currently unknown, however the attack was a suicide bombing, the report adds.
A explosion in the southern suburbs of Lebanon's capital Beirut has killed two people and wounded six, security sources told Reuters.
The area is a stronghold of support for the Shi'ite political and militant movement Hezbollah.
Iran's embassy in the same area was targeted by a bomb attack in November.