Two Russian embassy staff among six killed in Kabul suicide attack
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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.
Kremlin spokesman Dmitry Peskov, speaking to journalists in Moscow, slammed the attack as "absolutely unacceptable."
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 group.
As with other recent attacks, heavy Taliban security quickly sealed off the area and prevented media from filming nearby.
- 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.
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.
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.