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By Sue Pleming

WASHINGTON, May 21 (Reuters) - The United States voiced concern on Monday over fighting in Lebanon but said the country's security forces were justified in their attacks against Islamist militants in a Palestinian refugee camp.

The White House urged all sides to take a "step back" from the violence and said it was worried over civilian casualties.

"We are firm believers in Lebanon's democracy and sovereignty and support (Lebanese) Prime Minister (Fouad) Siniora's efforts to deal with fighting in the country," said White House spokesman Tony Fratto, who is in Crawford, Texas, with President George W. Bush.

At least 70 people have been killed in two days of violence and Lebanese tanks continued to shell militants in a Palestinian refugee camp near Tripoli in northern Lebanon on Monday.

The U.S. State Department said Lebanon's security forces were "rightfully" acting to enforce law and order.

"It would appear that the Lebanese security forces are working in a a legitimate manner to provide a secure, stable environment for the Lebanese people in the wake of provocations and attacks by violent extremists," State Department spokesman Sean McCormack told reporters.

McCormack said the militants, who come from the al-Qaeda-inspired Fatah al-Islam group, were committed to violence and the use of terror.

Asked whether he was concerned about the stability of Siniora's government in the face of the latest fighting, McCormack said Siniora was an "effective, strong" leader.

"Prime Minister Siniora's government has proven to be remarkably stable and resilient in the face of a number of political as well as violent provocations by those interested in undermining democratic progress in Lebanon," said McCormack.

"We have every confidence that he will act in the best interests of the Lebanese people dealing with these various challenges," he added.

The latest conflict is Lebanon's worst internal violence since the 1975-90 civil war.

Last summer, the United States was harshly criticized for giving Israel the green light to continue attacking Lebanon's Hezbollah guerrillas while others were calling for an immediate cease-fire in a war that lasted 34 days.

The United States provides assistance to Lebanon's security forces to help the country's fragile government as it faces a challenge from the Hezbollah-led opposition. The United States has designated Hezbollah a terrorist group

In recent months, the United States has delivered scores of new vehicles to Lebanon's security forces as well as riot control equipment and other gear as part of a $40 million military aid package.

Asked whether Lebanese security forces were getting any assistance from the United States in their attacks against Islamist militants, McCormack said the latest fighting was an "internal" Lebanese issue.

The United States has also pledged nearly $770 million in aid to rebuild Lebanon after last year's war with Israel. (Additional reporting by Steve Holland in Crawford, Texas)

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