By Lesley Wroughton

WASHINGTON (Reuters) - The United States and Germany told Russia on Friday that arming President Bashar al-Assad's forces could jeopardize international efforts for a peace conference on Syria.

Secretary of State John Kerry, speaking after talks with his German counterpart, Guido Westerwelle, said Russia's plans to deliver S-300 missiles to Syria were "not helpful" while trying to organize a peace conference to end the conflict. The missile shipment also posed a threat to Israel's security, he added.

Despite their differences, the United States and Russia are trying to convene an international conference next month to end a 26-month-old conflict that has killed more than 80,000 people and threatens to engulf more countries in the Middle East.

Washington is hoping that the conference, known as "Geneva 2" after a first conference last year in the Swiss city, will lead to a transitional government in Syria.

Russia has said it is committed to fulfilling a 2010 contract with Assad's government for S-300 long-range surface-to-air missiles as a deterrent against foreign military intervention. It slammed an EU decision this week to ease an embargo on arming the rebels - a move a European diplomat said was aimed at demonstrating to Assad that the West could eventually arm rebel forces.

Westerwelle told Russia that delivering the missiles "is totally wrong." He called on Assad "to stop the violence and come to the negotiating table" and urged the opposition to unite and participate in the meeting.

"It is very important that this Geneva 2 conference gets a realistic chance. And therefore, we ask and urge everyone not to spoil this conference," Westerwelle said, adding, "No one knows if this conference will become a success, but it is the wrong message which has been sent by Russia to the world and to the region by delivering S-300 or other weapons."

ACTING IN GOOD FAITH

While Kerry criticized Moscow's commitment to ship missiles to Assad, he also said Russia indicated it supported a transitional government in Syria.

"We will learn very quickly whether or not they and others are acting in good faith in an effort to provide legitimate names that might be acceptable to both sides who could provide that transitional government. And if they're not, the world will know it," Kerry said.

"Now, it is not helpful to have the S-300 transferred to the region while you are trying to organize this peace and create peace. It is not helpful to have a lot of other ammunition and other supplies going in, not just from the Russians, and they are supplying that kind of thing, but also from Iranians and Hezbollah."

Particularly worrying for the West is that the anti-aircraft missile system could make it far more dangerous to impose a no-fly zone, and could threaten planes deep inside the air space of Israel or NATO-member Turkey.

Israel, which has bombed Syria at least three times in recent months to prevent Assad from transferring arms to his Lebanese Hezbollah militia allies, has called on Moscow to scrap the S-300 deliveries.

"It has a profoundly negative impact on the balance of interests and the stability of the region, and it does put Israel at risk," Kerry said of the deliveries. "And it is not, in our judgment, responsible because of the size of the weapon, the nature of the weapon and what it does to the region in terms of Israel's security," he said.

IRAN PARTICIPATION

U.S., Russian and U.N. officials will meet in Geneva on Wednesday to prepare for the peace conference. The question of Iran's possible participation will be discussed, State Department spokeswoman Jen Psaki told reporters on Friday.

While the United States has not closed the door to Iran's participation in the conference, Psaki said Tehran had not played a constructive role in ending the violence in Syria.

Asked whether Iran should be part of the conference, Psaki said: "We have not made an evaluation of that. We want to work through the proper process with Russia, with the United Nations, to determine participation."

U.S. and European officials and intelligence agencies say Iran and Hezbollah, Iran's Lebanon-based ally, have stepped up support for Assad in the civil war.

A State Department report on Thursday found that Iran's sponsorship of terrorism overseas underwent "a marked resurgence" in 2012, reaching levels not seen in 20 years.

(Reporting by Lesley Wroughton; Editing by Peter Cooney)