Ukraine has activated its reserve forces and urged its nationals to leave Russia "immediately" following Russia's invasion of its eastern region on Monday.

In an evening speech to the nation, President Volodymyr Zelenskiy warned that Ukraine could face a battle for its existence but ruled out a national mobilization.

The decision to activate reservists was another indication that Ukraine was ready for a possible military confrontation with its neighbor.

The military stated that recruits with specialized talents, such as mechanics, will be sent to specialty units. Officials said that anyone who disregards the summons may face "criminal liability."

Around 900,000 people, according to experts, serve in Ukraine's reserve defense force.

Zelenskiy has advised his country to "increase the Ukrainian army's readiness for all possible operational changes." "We must rapidly replenish the Ukrainian army and other military formations," he stressed.

Zelenskiy's defiant address came after the United States announced that the invasion it had been forecasting for weeks had begun. According to Zelenskiy, Ukraine is a peaceful country, but its citizens will not remain silent in the face of attack.

"If we continue to be silent today, we will vanish tomorrow. Every day will be difficult, but we are prepared for it, confident in ourselves, our country, and triumph," he stated.

Ukraine appeals to its nationals to flee Russia.

Ukraine encouraged its citizens to leave Russia quickly on Wednesday, citing the danger of a Russian invasion complicating the supply of consular services.

"In light of Russia's escalating aggression against Ukraine, which may result in a significant reduction in possible consular assistance in Russia, the foreign ministry strongly advises Ukrainian citizens to avoid travel to Russia and those already there to leave immediately," the ministry stated.

Meanwhile, Maxar Technologies, a US-based space technology company, has published multiple satellite photos depicting what it claims are other military and equipment deployments in western Russia and over 100 vehicles in southern Belarus — all of which are located near Ukraine's borders.

Russia and Belarus have recently conducted massive exercises on Belarusian land. Neither has made a statement on the images.