Angry residents in northern Lebanon built a sand roadblock across a main highway that crosses into Syria on Thursday, a day after unidentified gunmen kidnapped two Lebanese farmers in the country's north and took them across the border into Syria.

A Lebanese security source said the farmers' relatives blocked other roads in the area to protest the kidnappings and urge the Lebanese government to help release them.

Northern Lebanon has been used as a sanctuary for Syrian refugees fleeing a government crackdown on a popular revolt against President Bashar al-Assad that has left thousands dead over the past 14 months.

Syrian forces, who have strong links with the Lebanese army and border guards, have at times crossed into Lebanon, shooting and arresting refugees and opposition rebels before retreating back across the poorly demarcated border, residents say.

Mohammed Yassin Merei and Mahdi Hamdan, from the border town of Abboudiyyeh in Akkar province, were seized by five armed Syrians while they were gathering crops, Lebanon's National News Agency said on Wednesday.

The Syria crisis has the potential to destabilise Lebanon, where some armed political factions feel they have an interest in Assad staying in power while others support the revolt, which started with peaceful protests in March 2011 but has grown increasingly violent. [Reuters]