Russian President Vladimir Putin said that Iran should be invited to an international meeting on the Syria conflict in Geneva at the weekend.
But the United States swiftly rejected the Russian proposal.
"It is better to involve Iran in the settlement (of the Syrian crisis)," Putin told a news conference following talks with Jordan's King Abdullah II on the shores of the Dead Sea.
"In any case it would complicate the process (if Iran is ignored)."
Wrapping up a one-day visit to Jordan, on a Middle East tour that had already taken him to Israel and the West Bank, Putin said Iran's support is needed.
"The more Syria's neighbours are involved in the settlement process the better. Ignoring these possibilities, these interests would be counterproductive, as diplomats say," he added.
"It is better to secure its support," said Putin.
The United States said, meanwhile, that it was increasingly likely to join the international meeting on the Syria crisis but rejected Russian calls for Iran to take part.
"We are getting closer but we don't have any decisions yet," State Department spokeswoman Victoria Nuland told reporters in Washington.
But concerning Iran she added: "Given its support for the regime and its continued behavior vis-a-vis Syria, we just don't see it as able to make a helpful contribution right now."
Russian Foreign Minister Sergei Lavrov earlier told reporters in Jordan that Iran should be invited to Saturday's meeting in Geneva.
"It should certainly be done," he said, adding that he would take part in the summit.
If Iran is not invited "then all those who can really influence all the main Syrian sides will not be present," he said.
UN-Arab League envoy Kofi Annan, who wants to gather the major powers in Geneva in a final bid to get agreement on a political transition plan for Syria, has yet to officially announce the meeting.
But Lavrov said he would still go, even if Iran is not invited.
"I will go. But in that case we will simply be talking about how to further gather together all the participants who are necessary in order to use this chance because there is no guarantee it will be crowned with success," he added.
"But this chance has to be used. For this to happen you need to gather all those who have real influence on the situation. Iran is no doubt one of them."
"One needs to agree to influence all Syrian sides so that they themselves sit down at the negotiating table and begin to get along and look for consensus solutions," he said.
"Only they themselves can find agreement, and outside players can help them get together."
It is not still clear if Britain, France and China -- the other three permanent members of the UN Security Council along with Russia and the United States -- will attend the Geneva meeting.
In Jordan, Putin opened a guesthouse for Christian pilgrims at Baptism site in the Jordan Valley, where many Christians believe Jesus was baptised.
He arrived in Jordan from the West Bank, where he met Palestinian president Mahmud Abbas during a brief visit to Bethlehem, a day after talks in Israel with Prime Minister Benjamin Netanyahu and President Shimon Peres.
"I did not set myself a task to negotiate a personal meeting between the two leaders, but I am under the impression that agreement is possible," between Israel and the Palestinian leaders, Putin said in Jordan.
The peacemaking Quartet, which groups Russia, the United States, the European Union and the United Nations, has been trying to nudge the two sides back to direct talks, on hold since late 2010.
But little progress has been made, as the Palestinians insist on a settlement freeze before talks resume, while Israel calls for new discussions without preconditions.
During his visit to Israel, his first since returning to the Kremlin for a historic third term in May, Putin was urged to act on Iran's controversial nuclear programme.
"The Iranian problem is not a simple one but still I proceed from the fact that it can and should be solved through absolutely peaceful means, by way of a negotiation process and on the basis of respect of the right of the Iranian people to the peaceful use of energy," Putin told reporters.
"With absolute guarantees provided to the world community that the realisation of this programme would not lead to the emergence of nuclear weapons in this country and will not facilitate the nuclear proliferation in the world."
The international community has been pursuing talks with Tehran, but three high-level meetings -- the most recent in Moscow -- have failed to produce any breakthrough.