The threat of action represents a change in stance from the Kremlin, as Russia initially chose not to respond to the steps taken by Obama.
The threat of action represents a change in stance from the Kremlin, as Russia initially chose not to respond to the steps taken by then outgoing US president Barack Obama in December.
The subject, however came up during US President Donald Trump's first face-to-face meeting with Russian leader Vladimir Putin last week.
Obama closed Russian facilities in New York and Maryland in response to purported cyber hacking attacks targeting parts of the US government, infrastructure, think tanks and political organisations.
The Russian diplomats and their families were given 72 hours to leave.
"If Washington decides not to solve this issue, we will have to take counter actions, and this is the rule of diplomacy, of reciprocity, of international affairs," Lavrov said in Brussels.
"We are still hopeful that the US, as a proponent of the rule of law, will finally respect international obligations," he added, during a press conference with EU diplomatic chief Federica Mogherini.
But the resurfacing of the expulsions row comes amid a much larger focus on alleged Russian meddling in last year's US election campaign, which is the subject of major investigations in Washington.
Speaking earlier in Moscow, Lavrov said Obama's "outrageous" move was designed "to poison Russian-American relations to the maximum and do everything to put the Trump administration in a trap."
"We are thinking about specific steps, and I don't believe that this should be discussed publicly," Lavrov told journalists of the potential diplomatic response.
Russia's deputy foreign minister Sergei Ryabkov, meanwhile, told the RIA-Novosti news agency that "there were several variants of a response and a harsh reaction is prepared."
The expelled diplomats were based in Washington and San Francisco.
Russian newspaper Izvestia said Monday, citing sources, that Moscow may expel 30 American diplomats and seize US property in the country.
Putin in December ruled out kicking out American diplomats, a move interpreted as Moscow's intention to build ties with a new White House administration, over the alleged cyber hacking, dubbed Grizzly Steppe by US officials.
Trump himself hailed Putin's restraint -- he even invited US diplomats' families to a party in the Kremlin -- as a "great move" and "very smart".
However, Russia wants to regain its properties in the United States and the subject was on the agenda of Putin's meeting with Trump at the G20 summit in Hamburg last week, according to the Kremlin.
Ryabkov reiterated Monday that "diplomatic property should be returned to us," RIA-Novosti reported.
Trump said he had pressed Putin over alleged meddling in the US election that catapulted him to power. He said the Russian leader "vehemently denied it."
In the latest twist to the election hacking row, however, Trump's eldest son Donald Jr on Tuesday released emails showing he embraced Russia's efforts to support his father;s campaign, saying he would "love" to get dirt from Moscow on Hillary Clinton.
The US Senate last month approved additional tough sanctions on Russia aimed at punishing Moscow for alleged election interference.
Lavrov warned that those steps threatened "the whole relationship" between Russia and the United States.
The sanctions led Moscow to cancel a meeting in June between US Under Secretary of State Tom Shannon with Russia's Ryabkov.
However, the two are reportedly planning to meet next week instead.