Russia and China will hold a large-scale military exercise next month that will include simulated nuclear weapons attacks, according to American defense officials.

The People's Liberation Army will send more than 3,200 troops, 900 pieces of military equipment, and 30 aircraft to Russia for the exercise known as Vostok-18, or East-18, the Chinese Defense Ministry said, noting the exercises will involve practicing maneuver defense, live firing of weapons, and counterattack.

"We urge Russia to take steps to share information regarding its exercises and operations in Europe to clearly convey its intentions and minimize and potential misunderstanding," Pentagon spokesman Eric Pahon said when asked about Vostok-18.

Additionally, the joint Russian-Chinese exercises scheduled for Sept. 11 through 15 will include military forces from Mongolia for the first time.

Mongolia, aligned with the Soviet Union until 1990, in recent years has sought closer ties to the United States, despite being located between Russia and China.

A Pentagon official said the Vostok war games will be closely watched by U.S. intelligence agencies because they are expected to include the simulated use of nuclear weapons. "It's their strategic messaging," the official said of both Russia and China.

"For nearly 20 years Vostok has been the Russian proving exercise for developing its new ‘escalate to deescalate' tactical nuclear doctrine involving the use of new, very small nuclear weapons fired mainly by artillery," said Rick Fisher, senior fellow at the International Assessment and Strategy Center.

Russia's new doctrine of rapidly escalating future conflicts by resorting to nuclear arms shortly after the outbreak of hostilities is a major concern for the Pentagon. The doctrine has been cited by Pentagon officials as one of the reasons Moscow was identified as a major strategic competitor in the new national defense strategy.

The doctrine is considered destabilizing and also is leading the United States to develop its own arsenal of smaller nuclear weapons.

The Pentagon revealed in its latest annual report on the Chinese military that China is also developing new, small nuclear weapons.

The participation of China in the nuclear war games also is a concern.

Based on the more than 3,000 troops and equipment going to Russia, Fisher said the training will "allow the PLA to learn a great deal about the Russian state of the art in combined warfare tactics, which is now an intense focus for PLA strategy development to prepare for war against Taiwan and the United States," Fisher said.

Both China and Russia are working to improve their ability to employ ground, naval, and air forces in joint operations over both short and long distances.

Russian Defense Minister Sergey Shoygu announced this week that the exercises would be the largest since the Soviet Union conducted a major command and staff exercise in 1981.

"This is the largest armed forces training event since the Zapad-81 maneuvers, it has acquired the status of an international exercise and is of unprecedented scale both in terms of spatial scope as well as the strength of military command and control entities, troops, and forces involved," Shoygu said, adding the PLA and Mongolian armed forces would take part.

The 1981 Zapad-81 war games involved more than 150,000 troops. Russia's last Vostok exercise in 2014 included more than 155,000 troops.

No details on the scenario for the war games were disclosed.

Shoygu said the exercises will be held in the Russian Far East and Siberia. The Chinese said the exercises will be centered at Russia's Tsugol training range in the Trans-Baikal region, located north of Mongolia's eastern border with Russia.

"Politically it is very significant," said Mark Schneider, a former Pentagon policymaker who specializes in Russian affairs.

"We have no details on the supposed scenario yet," he said. "However, in this type of exercise, the very fact of a joint exercise may be more important than the exact details of the announced scenario, which may be very real. Putin will probably give China what it wants out of the exercise. That could involve a threat to Taiwan."

Alexander Gabuev, senior fellow at the Carnegie Moscow Center, said the Chinese participation in Vostok-18 is significant.

"Vostok drills are aimed at countering foreign invasions and addressing military threats for Siberia and the Far East," Gabuev said in Twitter commentary. "China was among potential adversaries for many years. Now Moscow's message is that it doesn't view Beijing as an adversary any more."

Gabuev also noted that Chinese strategic bombers conducted patrols in the South China Sea and Taiwan Strait in July as part of Russia-hosted drills. It was the first joint strategic exercise by Beijing with a foreign air force.

The joint Chinese-Russian exercises come as the Trump administration has called both countries strategic competitors.

Wess Mitchell, assistant secretary of state for Europe and Eurasia, told a Senate hearing on Tuesday that Russia is the focus of a return to "big power competition."

"Contrary to the hopeful assumptions of previous administrations, Russia and China are serious competitors that are building up the material and ideological wherewithal to contest U.S. primacy and leadership in the 21st Century," Mitchell said.

"It continues to be among the foremost national security interests of the United States to prevent the domination of the Eurasian landmass by hostile powers."

Mitchell said effective policy toward Moscow must be backed by military power.

"To this end, the administration has reversed years of cuts to the U.S. defense budget, begun the process of recapitalizing the U.S. nuclear arsenal, requested close to $11 billion to support the European Deterrence Initiative, and, in the past year and a half, worked with NATO Allies to bring about the largest European defense spending increase since the Cold War—a total of more than $40 billion to date," he said.

The Communist Party-affiliated Global Times newspaper reported this week that Vostok-18 is the first time Chinese troops have taken part in the strategic exercise, and shows the deepening strategic partnership between Moscow and Beijing. The exercises will focus on "traditional security" rather than non-traditional threats, the report said.

"Chinese troops' participation in the Russian military drills shows that China-Russia comprehensive strategic partnership of coordination is further deepening in the military and security fields," the report said.

Igor Korotchenko, editor of the Russian magazine National Defense, was quoted in Global Times as saying the war games are a response to U.S. pressure on Russia and China and will seek to show that attempts to "contain" both countries will fail.

Video of Chinese tanks and military equipment likely to take part in Vostok-18 appeared on the internet in early August.

PLA tanks were shown transiting Kazakhstan on trains, prompting the Kazakhstan Defense Ministry to issue a statement saying the PLA arms were headed for a Russian military training site near Cherbarkul, a town some 900 miles east of Moscow in the Ural Mountains.

The Chinese forces will take part in another international military exercise to be held before Vostok-18 hosted by the Chinese-led, anti-U.S. alliance known as the Shanghai Cooperation Organization.

In a related development, Russian military forces in the eastern part of the country were placed on high alert on Monday ahead of the Vostok-18 exercises.

The alert was a five-day "snap inspection" in preparation for the upcoming international exercise, said Shoygu, the Russian defense minister.

Fisher, a China expert, said the Vostok exercises usually seek to test the latest offensive combined arms warfare tactics and new Russian weapons.

"If it indeed participates, the PLA will gain its first near-real exposure to modern combined arms warfare," he said.

The PLA participation in the Russian exercise likely will result in a reciprocal Russian military visit to China in the future. That would mean "the Russian force will be exposed to and likely be asked to critique the latest PLA developments in its combined-armed warfare tactics."

The joint military cooperation marks a significant increase in Chinese-Russian strategic cooperation and could lead to a formal military alliance, Fisher said.

"It also signifies that China is ready to provide political support at a minimum, and perhaps intelligence and maybe even logistic support, should Russia initiate aggression against the Baltic states, Poland, or the Ukraine," he added.

The joint military cooperation also may signify there is a developing Russian obligation to support China in the event it attacks Taiwan, or supports North Korea against South Korea and the United States," Fisher said.

The September 2014 Vostok exercise included 4,000 troops of the Russian Strategic Rocket Forces that are in charge of nuclear missiles.

The 2014 exercises prompted U.S. government concerns that Moscow was preparing for a large-scale intervention in Ukraine coinciding with the seizure of Ukraine's Crimean Peninsula that year.

The 2010 Vostok exercise took place on the Kamchatka Peninsula and reportedly practiced a Russian military intervention against China.