A suicide bomber struck Monday near the Russian embassy in the Afghan capital Kabul, killing two staff from the diplomatic mission and four others.
In the first attack targeting a foreign mission since the Taliban seized power in August last year, the bomber struck near the entrance of the embassy's consular section.
"Without any doubt, we are talking about a terrorist act, which is absolutely unacceptable," Kremlin spokesman Dmitry Peskov told journalists in Moscow.
Afghanistan's foreign ministry confirmed the deaths of two embassy staff.
Four Afghans waiting for consular services were also killed and several more wounded, Kabul police said.
Violence in Afghanistan has largely declined since the Taliban returned to power, but several bomb blasts -- some targeting minority communities -- have rocked the country in recent months, many claimed by the jihadist Islamic State (IS) group.
As with other recent attacks, heavy Taliban security quickly sealed off the area and prevented media from filming nearby.
No group has so far claimed the attack on the diplomatic mission.
Russian Foreign Minister Sergei Lavrov said immediate steps were taken to boost security at the embassy, located on one of Kabul's main roads and leading to the parliament building.
- Intelligence 'weakness' -
The attack is sure to embarrass the Taliban leadership, which for months has encouraged foreign nations to reopen their Kabul missions, insisting security was guaranteed.
During the chaotic takeover of the country by the Taliban last year, the Russian embassy was one of the few to remain open as most nations shut down and evacuated staff.
The Afghan foreign ministry said an investigation had been launched and authorities "will not allow the enemies to sabotage relations between both countries with such negative actions".
Afghan security analyst Hekmatullah Hekmat said the attack showed the government's "weakness" in gathering intelligence.
"If they can't prevent such attacks in the heart of Kabul, then they can't provide security in the countryside," he told AFP.
The United Nations mission in Afghanistan condemned the bombing.
"UNAMA stresses the need for the de facto authorities to take steps to ensure the safety and security of the people as well as diplomatic missions," it tweeted.
On Friday, a suicide bomber struck one of western Afghanistan's biggest mosques, killing at least 18 people, including its influential pro-Taliban imam. The cleric, Mujib ur Rahman Ansari, who had called for those who committed even the "smallest act" against the government to be beheaded, was killed in that attack in the city of Herat.
The attack against Ansari came despite authorities providing him with heavy security, including a bulletproof vehicle and bodyguards.
Several mosques across the country have been targeted this year, some in attacks claimed by IS.
At least 21 people were killed and dozens more wounded on August 17 when a blast ripped through a mosque packed with worshippers in Kabul.
IS has primarily targeted minority communities such as Shiites, Sufis and Sikhs.
While IS is a Sunni Islamist group like the Taliban, the two are bitter rivals and greatly diverge on ideological grounds.
Taliban officials claim that IS has been defeated but experts say the group is the main security challenge for the country's current Islamist rulers.