President Bush and German Chancellor Angela Merkel on Friday said Iran's nuclear weapons program "poses a grave threat to the security of the world" and called on it to back off its pursuit of the program.

"Iran armed with a nuclear weapon poses a grave threat to the security of the world," Bush said during a press briefing with Merkel.

Bush said he is not going to pre-judge how the United Nations Security Council will respond to the situation, but is preparing a strategy with allies.

Germany is a "valued ally" and the relationship between the two countries will be important in dealing with Iran and other issues, Bush said.

The United States, Germany and other nations need to "send a common message to the Iranians ... to not have a nuclear weapon to blackmail or threaten the world," Bush said.

Merkel said it is important to "persuade as many countries as possible to ally themselves with us" in showing opposition to Iran's nuclear weapons program.

"It is totally unacceptable about what Iran has said recently," Merkel said.

Bush told reporters in the East Room of the White House that they spent time talking about the issue and their shared desire to solve it diplomatically.

"We've got a lot in common," Bush said. "There's a lot of issues in the work that require our intense cooperation and desire to work together."

But Bush rejected a suggestion by Merkel that the U.S. prison camp at Guantanamo Bay, Cuba, be shut down.

The camp on the U.S. Navy base there is "a necessary part of protecting the American people," Bush said. Merkel said she raised the issue with Bush, and she described it as one of the differences between the United States and Germany.

"There sometimes have been differences of opinion, I mentioned Guantanamo in this respect," Merkel said.

Merkel has expressed criticism of the facility. Germany rebuffed an appeal by Attorney General Alberto Gonzales not to release a terrorist accused of killing a Navy diver in an airplane hijacking in 1985.

Merkel, recently elected to office in November, marked her first visit to the United States in an effort to improve her country's relations. She took power last November after an extremely close and protracted race with former German Chancellor Gerhard Schroeder.

The meeting with Bush follows a decision by European allies to demand answers from Iran about its nuclear weapons program.

On Thursday, Germany joined the United States, Britain and France in calling for the United Nations to look into Iran’s nuclear program.

Secretary of State Condoleezza Rice, who demanded on Thursday that the U.N. Security Council review Iran's "dangerous defiance," spoke by telephone Friday to Chinese Foreign Minister Li Zhaoxing. But at the United Nations, China's U.N. ambassador, Wang Guangya, said referring Iran to the Security Council might toughen Tehran's position on its nuclear program.

Rice did not say whether the United States has the votes needed in the U.N. Security Council to punish Iran formally. But Rice added that enough support has been shown for the International Atomic Energy Agency, which oversees nuclear activity, to ask the council to review the issue.

During her visit, Merkel will also meet with members of Congress and attend a ceremony at the newly renovated headquarters of the German Marshall Fund of the United States.

On Monday Merkel was due to fly to Moscow to meet with Russian President Vladimir Putin. Iran was likely to be on the agenda, along with Russia's conflict with Ukraine over shipping natural gas to Europe.

FOX News' Carl Cameron and The Associated Press contributed to this report.