President Reagan agreed yesterday to earmark an additional $30 million for civilian relief in Lebanon, bringing to $65 million the total committed by the United States to help restore the war-devastated Middle East country.

Reagan agreed to the additional funds after listening to Peter McPherson, administrator of the Agency for International Development, describe the damage he saw in a three-day trip to Lebanon earlier this month.

In a news conference after meeting with the president, McPherson said he was "greatly distressed at the intensity and extent of damage to housing and life-support infrastructure facilities such as power, water supply, urban streets and sanitation facilities" in Lebanon.

McPherson described Damour, a coastal city of about 25,000 south of Beirut, as "a ghost town--you walk through it and there's nobody there." He said a refugee camp outside the southern city of Sidon that previously housed about 25,000 Palestinians is "now basically . . . almost leveled."

McPherson accused Israel of periodically cutting off food and medicine to besieged west Beirut, despite previous statements by Israeli Prime Minister Menachem Begin that such supplies would be allowed to reach the estimated 400,000 civilians in the sector.

McPherson said the United States is exerting diplomatic pressure on the Israeli government to allow passage of supplies, and he predicted that Israel would comply.

"We're making it very clear that we think the Israelis must let this food in," said McPherson, who added that Samuel Lewis, U.S. ambassador to Israel, is communicating the U.S. position "to the highest level of the Israeli government."

McPherson also disputed statements by Israeli officials that only 10 buildings had been damaged irreparably in Sidon. While the word "irreparably" may be subject to dispute, McPherson said, "I saw many, many buildings that I wouldn't personally want to foot the bill to repair."

He suggested that Israeli and Lebanese officials have been issuing incorrect figures for civilian casualties. "I don't think there is a sound basis for any statistics at this time," he said, noting that civilians in the war zone have been dispersed widely.

In some areas, such as the port city of Tyre, he said, Israeli forces allowed civilians sufficient time to evacuate contested zones, while in Sidon, "The civilian population wasn't evacuated, and casualties were greater."

Nachman Shai, an Israeli Embassy spokesman here, said Israel agreed last week to lift its blockade on food and medical supplies for humanitarian reasons, not because of U.S. pressure. He said the Israeli government stood by its claim on damage in Sidon, but he conceded that its casualty estimates for civilians will need readjustment after the fighting.

Excluding those in west Beirut, McPherson said, most civilians in Lebanon appeared to have enough food and medical supplies for the immediate future. Even the hundreds of thousands of homeless were able to survive outdoors during the warm summer months, he said, although he predicted increasing problems as personal savings are depleted and cold weather sets in.

Reagan originally announced a commitment of $15 million in U.S. relief funds for Lebanon, and asked Congress for an additional $20 million. Congress authorized $50 million, and the president has decided to use the full amount.

McPherson said most of the money would be used to repair roads, power plants and communications facilities heavily damaged in the five-week-old conflict. He said he is confident that the Lebanese can rebuild given economic help. "They certainly want to put their own house in order," he said.

The United States has pledged $2 million to the U.N. Relief and Works Administration and $1.5 million to the U.N. Children's Fund, and has made cash grants of $2.8 million to other groups, including $600,000 to the International Red Cross.