President Trump's top White House expert on Russia won't attend his meeting with Russian President Vladimir Putin.

Trump and Putin will be joined only by Secretary of State Rex Tillerson and Russian Foreign Minister Sergey Lavrov, as well as translators, according to reports. That leaves a leading Kremlin critic outside the first face-to-face meeting of the two leaders, which Tillerson prefaced by outlining a proposal for significant cooperation in Syria between the United States and Russia.

"The United States and Russia certainly have unresolved differences on a number of issues, but we have the potential to appropriately coordinate in Syria in order to produce stability and serve our mutual security interests," Tillerson said Wednesday evening.

He revealed only modest aspirations for the meeting itself, which takes place on the sidelines of the G-20 summit in Hamburg, Germany. "It's difficult to say what Russia's intentions are in this relationship," Tillerson told reporters aboard his plane, adding that he hoped the meeting would feature "a good exchange between President Trump and President Putin over what they both see as the nature of the relationship between our two countries."

Trump's team reportedly considered bringing Fiona Hill, who handles Russia and European policy issues for the National Security Council, but that proposal did not come to fruition. "If she [Hill] wasn't there it would be pretty bad, this is the most momentous thing in her portfolio," Evelyn Farkas, a deputy assistant secretary of defense in the Obama administration, told the Daily Beast.

Russia has taken foreign policy positions diametrically opposed to U.S. interests in a range of areas, which could compromise even efforts to cooperate to consolidate the defeat of the Islamic State in Syria. Russia has worked with Iran to prop up Syrian President Bashar Assad and secure territory for Iran that would expand the Shia regime's ability to coordinate with terrorists in the Middle East, to the alarm of U.S. leaders. Tillerson, in recognition of this dynamic, warned against such Russo-Iranian collaboration in the plan he outlined Wednesday.

"The United States believes Russia, as a guarantor of the Assad regime and an early entrant into the Syrian conflict, has a responsibility to ensure that the needs of the Syrian people are met and that no faction in Syria illegitimately re-takes or occupies areas liberated from ISIS' or other terrorist groups' control," Tillerson said.