Human rights group Amnesty International on Friday accused the United States of having a "lopsided" response to the crisis in Gaza and told the top U.S. diplomat to press all sides to reach an immediate cease-fire.

In a letter to U.S. Secretary of State Condoleezza Rice, Amnesty's U.S. branch said the U.S. government must not ignore Israel's "disproportionate response" against Gaza and policies the group said had brought Hamas-ruled Gaza to the brink of "humanitarian disaster."

"Amnesty International USA is particularly dismayed at the lopsided response by the U.S. government to the recent violence and its lackadaisical efforts to ameliorate the humanitarian crisis in Gaza," the group told Rice in the letter, which was released to the media by Amnesty.

Amnesty said it was also deeply concerned by weaponry and military equipment supplied to Israel by Washington that the group said had been used in recent strikes on densely populated residential areas in Gaza.

"The United States must suspend the transfer of weapons to Israel immediately and conduct an investigation into whether U.S. weapons were used to commit human rights abuses," said Amnesty.

The United States came under heavy criticism during Israel's 2006 war with Lebanon for supplying cluster bombs to Israel that were used in the conflict.

The United Nations estimates that Israel dropped a few million cluster bombs on Lebanon. Hundreds of thousands of those bomblets failed to explode and have continued to maim and kill after that war's end.

Amnesty also urged the United States to put pressure on Israel to open border crossings into Gaza to human rights workers and journalists and to allow humanitarian aid through.

A U.S. State Department official who requested anonymity rejected claims that the U.S. response had been pro-Israeli or lopsided.

Asked about claims Israel was using disproportionate force, he said: "The use of force is based on military objectives. Israel has never claimed that this is vengeance. They are looking to end Hamas' ability to attack Israel."

Israel launched an air campaign a week ago against Gaza in retaliation for rocket attacks by Hamas deep into Israel. So far the air strikes have killed at least 425 Palestinians while rockets fired from Gaza have killed four Israelis.