NATO Secretary-General Jens Stoltenberg demanded that Russia de-escalate its conflict with neighboring Ukraine on Wednesday in a bid to resolve the crisis politically.

- We call on Russia to de-escalate and reduce tensions (with Ukraine) and to be transparent about their intentions - Stoltenberg told the Berlin-based Friedrich-Ebert Foundation via video link. 

- We really hope that this time Russia will not use military force but respect the sovereignty and territorial integrity of a sovereign and independent state in Europe – Ukraine.-

Stoltenberg warned of a "very serious situation" amid the escalating Russian-Ukrainian conflict.

He accused Moscow of "not wanting Ukraine to be an independent and sovereign nation."

The NATO chief reiterated the "need for a political solution" in the crisis.

Russia, Ukraine and NATO have stepped up military exercises as tensions have intensified in recent weeks as Kyiv has openly accused Moscow of planning to stage an invasion.

Russian military aircraft were scrambled Wednesday to escort French Rafale and Mirage fighter jets flying above the Black Sea, according to the RIA news agency that cited the Russian Defense Ministry.

Ukraine alleges that more than 14,000 people have been killed in seven years of fighting with Russian-backed separatists.