The United Nations says it may step up efforts to clear land mines from South Sudan following a mine explosion this week that killed 20 people.

The U.N. says 18 civilians and two soldiers were killed on Sunday when the bus in which they were travelling struck an anti-tank mine recently laid by rebels in the area.

The explosion, which happened in the northern state of Unity, also injured seven people. U.N. officials say it was the deadliest mine explosion ever in South Sudan.

A U.N. official said the road on which the bus was travelling was known to contain unexploded land mines, but civilians continued to use it because of a lack of alternate routes.

Since declaring independence from Sudan in July, South Sudanese authorities have clashed with dissident rebel militias in the country.

The rebels are suspected of laying anti-tank and anti-personnel mines across a number of areas in Unity state.

The United Nations currently has about 15 people in the area working on mine clearance.