Kim Jong Un received a message from Chinese President Xi Jinping Wednesday, reportedly sent “out of politeness.” It was the first time Xi initiated communication with the North Korean leader in over a year, according to The Telegraph.

The letter was a response to a message from Kim in which he offered  Xi “sincere congratulations” for beginning a second term as president. Xi reportedly referred to Kim in the message as “Comrade Chairman.”

“I wish that under the new situation the Chinese side would make efforts with the DPRK side to promote relations between the two parties and the two countries to sustainable soundness and stable development,” Xi said, according to North Korean state news agency KCNA.

The message could mean recently strained relations between the two nations are improving, experts said. Kim’s previous note to Xi said he believed the countries relationship would develop “in the interests of the people of the two countries.”

“The fact that both sides are swiftly trading letters and announcing it carries a symbolic meaning,” Professor Yang Moo-Jin of the University of North Korean Studies in Seoul told Agence France Presse. “If the message was more intimate, we could expect a faster thawing of ties. But for now, it shows that both sides agree on the need to improve their relations.”

Xi has led China since 2012 and was recently confirmed for a second five-year term. While China has backed North Korea since the 1950s, the relationship between the two countries has become tense as China backed United Nation’s sanctions against Kim’s regime. President Donald Trump’s administration has called on China to put increasing pressure on North Korea to end its nuclear program.

“I am very disappointed in China,” Trump said in a series of July tweets. “Our foolish past leaders have allowed them to make hundreds of billions of dollars a year in trade, yet they do NOTHING for us with North Korea, just talk. We will no longer allow this to continue. China could easily solve this problem!”