Israel unleashed an artillery bombardment on Gaza today for the first time in its week-long offensive, prompting increased speculation that a ground invasion is about to begin.

Palestinian medical officials also said that an Israeli airstrike on a Gaza mosque had killed 10 people and wounded dozens more. Al Jazeera quoted witnesses as saying there were at least 200 people at prayer inside the Ibrahim al-Maqadna mosque in northern Gaza when the missile struck.

The Israeli TV station Channel 10 said the entire length of the Gaza Strip was under attack. Palestinian witnesses told Reuters the shelling had caused a large explosion in Gaza City and there were a series of blasts close to the frontier with Israel. There were no immediate reports of casualties.

Israeli TV commentators said shelling could be used to clear Hamas-laid minefields ahead of a possible ground invasion. Israeli troops are massed on the border waiting for orders to go in.

Speaking from Syria, the exiled Hamas leader, Khaled Mashaal, warned that any ground assault would lead Israel to "a black destiny of dead and wounded".

Sustained artillery fire is likely to significantly increase the number of civilian casualties, according to a report on the website of the Israeli newspaper Ha'aretz.

Israel's first artillery use came shortly after Gordon Brown telephoned his Israeli counterpart, Ehud Olmert, urging him to stop the attacks.

"The prime minister has spoken again today to Prime Minister Olmert, and is pressing hard for an immediate ceasefire," a Downing Street spokesman said. "We are working urgently with international partners to address the underlying causes of the conflict, including trafficking of arms into Gaza ... Moderation must prevail."

Earlier today,Israeli aircraft and gunboats continued attacking targets in Gaza. The Israelis struck the homes of two Hamas operatives, saying the buildings were used to store weapons and plan attacks. Hamas said one of its military leaders, Abu Zakari al-Jamal, was killed in an air raid last night.

More than 400 Palestinians and at least four Israelis have been killed since Israel launched its offensive last Saturday. The UN estimated yesterday that a quarter of the Palestinians killed were civilians.

George Bush today blamed the continuing violence on Hamas terrorism and offered no criticism of Israel in his first comments since the offensive began a week ago.

The US president condemned Hamas's campaign of rocket attacks on Israel as an "act of terror" and said no peace deal would be acceptable unless the flow of smuggled weapons to terrorist groups was monitored and stopped.

"This recent outburst of violence was instigated by Hamas, a Palestinian terrorist group supported by Iran and Syria that calls for Israel's destruction," he said.

The White House will not comment on whether it views the Israeli response as proportionate.

In his weekly radio address, Bush said Washington was "leading diplomatic efforts to achieve a meaningful ceasefire that is fully respected".

He said he would keep the president-elect, Barack Obama, updated on the negotiations. Obama has refused to comment on the crisis while Bush remains in office.

The UN secretary general Ban Ki-moon last night urged world leaders to intensify efforts to achieve an immediate ceasefire.

Israel has said it will accept a ceasefire only if it is enforced by international monitors.

Palestinian militants continued to fire rockets into southern Israel today. There were no injuries, though one rocket hit a house in the southern city of Ashkelon, police said.