BEIRUT, Lebanon — Israeli troops clashed with Hezbollah guerrillas on the Lebanese side of the border today, while warplanes flattened 20 buildings and killed at least 19 people, officials said, as fighting entered its second week.

Hundreds of Americans boarded a luxury ship at Beirut's port that was to carry them from the country, with many complaining about the slow pace of the U.S. evacuation effort. Europeans and Lebanese have fled by the thousands.

Hezbollah's Al-Manar TV, meanwhile, reported that the Islamic militant group struck an Israeli air base 30 miles from the Lebanese border, but the Israeli military denied any of its bases had been hit. That distance would make it the deepest strike by Hezbollah into northern Israel in more than a week of fighting.

Israeli bombers, which had been focusing on Hezbollah strongholds in southern Beirut, also hit a Christian suburb on the eastern side of the capital for the first time. The target was a truck-mounted machine that was used to drill for water but could have been mistaken for a missile launcher. The vehicle was destroyed, but nobody was hurt in that attack.

Military officials said Israeli troops crossed the border in search of tunnels and weapons. Hezbollah claimed to have "repelled" Israeli forces near the coastal border town of Naqoura. Casualties were reported on both sides.

The Israeli army confirmed there were clashes with Hezbollah in the border area and that some Israelis had suffered casualties. The army would not elaborate. Hezbollah's Al-Manar television channel reported that two Israeli soldiers had been killed and three wounded, but that could not be confirmed.

Israel, which has mainly limited itself to attacks from the air and sea, had been reluctant to send in ground troops because Hezbollah is far more familiar with the terrain and because of memories of Israel's ill-fated 18-year occupation of south Lebanon that ended in 2000.

The fighting dealt a blow to diplomatic efforts to broker a cease-fire and to send a new international force to bolster the 2,000-member U.N. force in south Lebanon appeared stalled.

Israel declared Tuesday it was ready to fight Hezbollah guerrillas for several more weeks, raising doubts about international efforts to broker an immediate cease-fire. The fighting has killed nearly 300 people and displaced 500,000.

"It will take us time to destroy what is left," Brig. Gen. Alon Friedman, a senior army commander, told Israeli Army Radio today.

Israel stressed it did not plan to target Hezbollah's main sponsors, Iran and Syria, during the current fighting.

"We will leave Iran to the world community, and Syria as well," Vice Premier Shimon Peres told Army Radio. "It's very important to understand that we are not instilling world order."

The U.N. children's and health agencies said today they were concerned about civilian casualties and new health risks because of escalating violence in Lebanon and Israel.

"Civilian deaths include dozens of children, with many more injured," the joint statement said. "The psychological impact is serious as people, including children, have witnessed the death or injury of loved ones and destruction of their homes and communities," UNICEF and the World Health Organization said.