Heads of state rarely send thousands of their soldiers to fight in someone else’s war without expecting something in return.

So the decision by Kim Jong-un, the leader of North Korea, to deploy 3,000 troops to the battlefield in Ukraine has left American officials with several urgent questions: What is President Vladimir V. Putin of Russia giving Mr. Kim? Could Russia help North Korea develop more lethal missiles and nuclear capabilities? And could the exchange be evidence of a dangerous new military alliance?

National security and intelligence officials in the United States said Wednesday that the answers to those questions remained murky, even as they released evidence showing that the troops had been transported by ship from the North Korean port city of Wonsan to Vladivostok in Russia.

John Kirby, the national security spokesman at the White House, said intelligence officials had found no evidence of a specific promise by Russia to help bolster the North Korean military. But he told reporters that there was plenty that Russia could do to help Mr. Kim: “That’s what’s so concerning to us.”

Analysts and experts who have spent decades tracking the military efforts of Pyongyang say Mr. Kim is most likely seeking help from Russia in two broad categories: short-term assistance with his military capabilities and longer-term strategic assurances that could bolster Mr. Kim’s ability to confront the United States and his neighbors.

“There is no stronger signal that one country can send to another than sending troops into the battlefield,” said Victor D. Cha, a professor of government and international affairs at Georgetown University and the Korea chair at the Center for Strategic and International Studies in Washington. Dr. Cha said sending the troops would give the North Korean leader leverage to ask Russia for a lot in return.