Tensions mount in Ukraine crisis; see pics

Ukrainian service members guard an area on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine

Service members of the Ukrainian armed forces stand guard at combat positions near the line of separation from Russian-backed rebels in the town of New York in the Donetsk region, Ukraine.

A Ukrainian service member guards an area on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

A Ukrainian service member guards an area as dogs look on at the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

A Ukrainian service member guards an area on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

Ukrainian service members guard an area on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

A Ukrainian service member guards an area on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

Ukrainian service members walk on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

A Ukrainian service member walks on the front line at the industrial zone of government-held town of Avdiyivka in Donetsk region, Ukraine.

A service member of the Ukrainian armed forces patrols the area at combat positions near the line of separation from Russian-backed rebels in the Donetsk region, Ukraine.