US Secretary of State Antony Blinken has questioned Kazakhstan's decision to seek Russian military aid to deal with an ongoing wave of violent unrest, BBC reports.
Dozens of people have been killed in protests triggered by a rise in fuel prices, but Mr Blinken said the US believes that the Kazakh government can deal with the protests itself.
He told reporters that it was unclear why the deployment was happening.
The first of about 2,500 Russian-led troops have arrived in Kazakhstan.
Officials in Moscow have emphasised that the deployment of its forces under the Collective Security Treaty Organization (CSTO), a Eurasian military alliance of five former Soviet republics and Russia, is temporary.
President Kassym-Jomart Tokayev made the request for assistance after protesters stormed the mayor's office in Kazakhstan's largest city, Almaty, and overran the city's airport.
But addressing reporters at a State Department briefing, Mr Blinken warned that "one lesson of recent history is that once Russians are in your house, it's sometimes very difficult to get them to leave".
"It would seem to me that the Kazakh authorities and government certainly have the capacity to deal appropriately with protests to do so in a way that respects the rights of protesters while maintaining law and order," Mr Blinken said.
"So it's not clear why they feel the need for any outside assistance. So we're trying to learn more about it."
Meanwhile, the US has authorised the departure of some non-essential staff from its consulate in Almaty amid safety concerns over the ongoing protests.
Some Russian paratrooper units have already arrived in the country, and on Friday assisted Kazakh forces in retaking the airport from protesters.
Kazakh troops have also taken firm action to regain control in Almaty. On Thursday, local media published videos showing government troops opening fire on protesters.
The Interior Ministry says 26 "armed criminals" and 18 security officers have been killed so far in the clashes and President Tokayev blamed what he called foreign "terrorists" for the unrest.
