Amnesty International has slammed the US for its 'lopsided' support of Israel's attacks on Gaza which has created a humanitarian crisis.

"Amnesty International USA is particularly dismayed at the lopsided response by the US government to the recent violence and its lackadaisical efforts to ameliorate the humanitarian crisis in Gaza," the rights group said in a letter to Secretary of State Condoleezza Rice.

At least 436 Palestinians, including women and children, have been killed and 2,280 wounded since Israel began its air and sea attacks on Gaza Strip. Rockets fired from Gaza have killed six Israelis and injured 16.

Amnesty's US branch further urged Washington to suspend weapons deliveries to Israel, which has used them to attack densely populated residential areas in Gaza.

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

The White House deputy press secretary Gordon Johndroe defended on Friday Israel's attack on Gaza, saying a ceasefire is based on Israeli's demand for Hamas to halt rocket attacks.

"Israel has a right to defend itself from these rocket attacks, and so we'll see," Johndroe told reporters when asked about progress toward a ceasefire.

Amnesty called on Washington to put pressure on Israel to open up border crossings into Gaza to human rights workers and journalists and to allow humanitarian aid through.

A US State Department official, speaking on condition of anonymity, rejected claims that the US response had been lopsided.

"The use of force is based on military objectives. Israel has never claimed that this is vengeance. They are looking to end Hamas's ability to attack Israel," he said.

Gazans are facing starvation and the shortage of food and medical supplies. The main power plant in the strip has been knocked out, making it impossible for the overwhelmed hospitals to properly treat victims and provide them with essential medical services.