NATO Secretary General Jens Stoltenberg has warned of a risk of "incidents and accidents," following Sunday's airstrikes by Russia on a Ukrainian military facility in western Ukraine. The facility is located near the border with Poland.

The NATO chief made the remark at a news conference on Tuesday.

He said, "When we see more military activities, when we see fighting going on close to NATO borders, there's always a risk of incidents and accidents."

He also said NATO is very closely monitoring its airspace and border areas.

Asked about concerns over the use of chemical weapons by Russia, Stoltenberg said he "will not speculate about any military response from the NATO side." But he added that "NATO's main responsibility is to make sure that we defend and protect all Allies."

He revealed that NATO will hold an extraordinary summit meeting of member nations next week at its headquarters in Brussels, Belgium.

He said participants will discuss support for Ukraine as well as plans to strengthen defenses of member states.