Russian President Vladimir Putin on Thursday blasted Washington for being "unconstructive" on Syria after the Kremlin said the United States had declined to host a high-level delegation from Moscow.
"I believe this is an unconstructive position," Putin said on a visit to Kazakhstan, where he also called for countries with large Muslim populations to join an alliance against terrorism.
"I don't quite understand how our American partners can criticise Russia's actions in Syria in the fight against international terrorism if they refuse to hold direct dialogue even in such an important area as political settlement."
"What apparently lies at the heart of the weakness of the American position is a lack of any kind of agenda," Putin said in televised remarks.
"It looks like they have nothing to talk about."
"Nevertheless, we are keeping our doors open and very much hope for dialogue with all the participants of this complicated process, including with our American partners."
Earlier this week Putin said he was willing to send a high-ranking delegation to the United States led by Prime Minister Dmitry Medvedev.
He also accused Washington of seeming not to know what its goals were in Syria.
"I believe some of our partners simply have mush for brains," Putin said this week.
Foreign Minister Sergei Lavrov said Thursday Washington had declined to host a Russian delegation and also refused to send its own mission to Moscow.
Washington said it was not interested in having any talks while Moscow keeps bombing Western-backed moderate rebels in Syria under cover of its fight against IS jihadists.
"We've said that we're not interested in doing that as long as Russia is not willing to make a constructive contribution to our counter-ISIL effort," White House spokesman Josh Earnest said.
"Russia has their own agenda, and it's an agenda right now that they're pursuing on their own."
Putin at the weekend said the goal of Moscow's air campaign was to "stabilise the legitimate authorities", while Washington has repeatedly said Syria's embattled leader Bashar al-Assad should go.
Speaking after talks with President Nursultan Nazarbayev of Kazakhstan, Putin praised his counterpart's call for an "Islam against Terrorism" forum that would see Muslim countries jointly tackle terrorism.
"Of course we support the proposal of the president of Kazakhstan to unite the efforts of all countries, foremostly those with populations that observe Islam, in the battle against terrorism," said Putin.
Nazarbayev, whose majority Muslim country has seen its nationals join the ranks of IS militants in Syria, appeared to lend support to ally Russia's bombing campaign.
"Currently there is a lot of speculation surrounding the subject of Sunnis and Shias, but this is not relevant, because what is happening in Syria is a threat to all of us," Nazarbayev said.
Putin's bombing campaign and his alliance with Shiite Iran and the Shiites in Iraq have angered many Sunni Arabs who accuse Assad of targeting his country's Sunni majority.
According to the International Crisis Group, between 2,000 and 4,000 fighters from Central Asia may be fighting alongside IS jihadists, mostly in Syria, stoking fears of radicalised citizens returning home one day.
Moscow says it needs to target IS jihadists before they cross into Russia, which also has a large Muslim population.