KFAR SHOUBA, Lebanon - Four explosions and gunfire were heard on Friday in the disputed Shebaa Farms area, Lebanese witnesses said.

It was not clear what caused the blasts, which sparked a fire on an uninhabited hillside and raised tensions on the Lebanese frontier two days after a rocket fired from south Lebanon landed in a northern Israeli town, damaging a building but causing no casualties.

Two explosions were also heard overnight near Israeli positions in the Shebaa Farms, a strip on the border between Lebanon, Israel and Syria's Israeli-occupied Golan Heights.

There have been no claims of responsibility for any of the blasts and Lebanon's Hizbollah guerrilla group, which controls the southern border area, had no comment.

A spokeswoman for Kofi Annan said the U.N. secretary-general was "gravely concerned" about the reports and urged Lebanon to put an end to all such attacks.

"The Secretary-General urges all parties to fully respect the Blue Line and reminds them that one violation of the Blue Line cannot justify another," she said, referring to the U.N.-drawn line marking the Israeli withdrawal from Lebanon.

The Jewish state ended a 22-year occupation of southern Lebanon in 2000, under pressure from Shi'ite Muslim Hizbollah.

Palestinian militants have launched rocket attacks from across the border in recent years. A previous attack in November prompted Israeli threats of retaliation.

The United Nations has certified Israel's withdrawal from Lebanon as complete and says the Shebaa Farms is Israeli-occupied Syrian territory. Lebanon and Syria say the area is still-occupied Lebanese soil. (Reuters)