Russia will be forced to protect the residents of Donbass if Ukraine launches full-scale hostilities against the region, said Russian President Vladimir Putin's deputy chief of staff on Thursday.

Dmitry Kozak claimed that a ramping-up of the conflict would lead to the end of Ukraine, with the Kremlin forced to stand up for its citizens living in the territory of Donbass.

"Everything depends on what the scale of fighting will be. If there is, as the president says, a Srebrenica, we will be forced to stand up for ourselves," Kozak told RT, referring to Putin's comment in 2019 that he fears a genocide in Donetsk and Lugansk, if Kiev regains control of Donbass.

Ukraine news agency Interfax reported that a U.S. delegation led by Defense Attache Colonel Brittany Stewart paid a working visit to the Joint Forces Operation (JFO) area on Friday.

According to the press center of the JFO headquarters, U.S. representatives visited the units of the Armed Forces of Ukraine, talked with Ukrainian defenders and once again made sure of the presence of Russian mercenaries in Donbass.

Many Ukrainian citizens applied for a Russian passport after Moscow simplified the application process in 2020. 

Russia now has more troops on Ukraine's eastern border than at any time since 2014, according to the U.S. White House Press Secretary Jen Psaki.

Ukrainian President Volodymyr Zelensky visited Donbass earlier due to the escalation of tensions in the region. Zelensky inspected the positions of the Ukrainian armed forces at the frontline of defense, where the largest number of ceasefire violations were recorded.

Zelensky noted that 26 Ukrainian soldiers have been killed in the conflict in Donbass since the beginning of this year.

Kozak commented on Zelensky's visit as "playing with fire."

On Tuesday, Ukraine's Zelensky urged North Atlantic Treaty Organization (NATO) to speed up his country's membership plan, saying it is the only way to end a simmering conflict in Donbass after a phone call with NATO Secretary-General Jens Stoltenberg.

The Russian Foreign Ministry on Wednesday called on Kiev and the NATO to stop its aggregations along the Russia-Ukraine border. 

The NATO continues financial and logistical support for the Ukrainian armed forces, the supply of lethal weapons and the training of Ukrainian military personnel. All this does not contribute to the security in the region or the settlement of the conflict in Donbass, but causes Russia's serious concerns, the spokesperson said.

Read more: Putin, Merkel 'concerned' over east Ukraine tensions: Kremlin

A ceasefire was announced in eastern Ukraine on July 27 last year. But the tension has been aggravated between the Donbass insurgents and Ukrainian government forces escalated since this year.

The ongoing conflict in eastern Ukraine, which began in April 2014, has claimed some 14,000 lives and left as many as 40,000 wounded. The separatists, which are close to Moscow, now controlling large swathes of territory on Donbass. 

(With input from agencies.)