Several people have been injured in a shootout in central Kyiv between two branches of Ukraine’s security services, in an incident Volodymyr Zelenskyy described as “disgraceful”.

The shootout, which took place on Wednesday morning in a downtown area of the capital, was between officers from the domestic security service, the SBU, and the military intelligence directorate, the HUR.

The SBU said in a statement on Thursday that the shootout “definitely shouldn’t have happened” and that it was assisting state investigators.

The clash appears to have been sparked by the SBU’s recent arrest of a Russian man fighting for Ukraine under the supervision of the HUR. Local media reported that while the SBU was searching an apartment linked to the investigation, HUR officers arrived and shots were fired; three HUR officers were injured.

The SBU said that, together with the HUR, it had foiled a Russian-backed terrorist plot targeting a Russian opposition leader who had recently arrived in Ukraine. In a separate statement, the SBU said its officers had come under armed attack while conducting an investigation and they had used their weapons in response.

On Thursday, the SBU released a video of the Russian national fighting for Ukraine who had been arrested last month. In the video, the man says he was contacted by a person from Russia acting for Moscow’s intelligence agency who wanted to recruit him to assassinate Ukrainian defence personnel and other individuals. The man’s lawyer told local media that the video had been recorded under pressure and was being released to “whitewash” the shootout. The man has since been released from SBU custody.

Hours after the shootout, Zelenskyy fired the SBU’s head of the department for the protection of state secrets, Oleh Khramov.

“Shooting here in Ukraine is permitted only at Russian occupiers, in defence of Ukraine. Our state will never regard any other shootout as acceptable,” Zelenskyy wrote in a social media post.

The incident comes at a delicate moment for Zelenskyy. He has recently faced down calls from a sacked defence minister to hold wartime elections. The demand, from Mykhailo Fedorov – whose dismissal triggered a wave of protests – was the biggest challenge to Zelenskyy’s authority from inside Ukraine since the start of the Russian invasion.

Zelenskyy also recently sacked a presidential deputy chief of staff amid reports she was being investigated in connection to an alleged money-laundering scheme.

The president said the shooting on Wednesday needed to be investigated by both the SBU and the HUR. Ukraine’s main investigative agency has also opened an investigation.

In a separate incident, a 71-year-old man was shot outside a medical clinic on Thursday morning in Kyiv and remains in a “serious condition”, according to the the Kyiv city prosecutor’s office. An investigation has been launched into an attempted murder.

The shootings come as Kyiv withstands an intense period of Russian bombardment. Air raid alerts have been active for extended periods in the daytime as well as overnight in the capital. Across the country, at least five people were killed in strikes on Wednesday night. Ukraine is facing a critical shortage of US-made ​Patriot interceptors capable ⁠of shooting down Russian ballistic missiles.

Ukraine has also increased its use of long-range drones to target deep into Russian territory. This week Zelenskyy said that Russian airspace was becoming unsafe for commercial airlines because of the growing number of drone attacks.