U.S. President Donald Trump on Saturday expressed hope that Japan will contribute more to its alliance with the United States as he issued a statement marking the 60th anniversary of the signing of the two countries’ security treaty.

“As the security environment continues to evolve and new challenges arise, it is essential that our alliance further strengthen and deepen,” Trump said. “I am confident that in the months and years ahead, Japan’s contributions to our mutual security will continue to grow, and the alliance will continue to thrive.”

Trump has been calling on Japan to pay a larger proportion of the cost of stationing U.S. troops in the Asian country, while being critical of what he perceives as the one-sided nature of the security treaty. The alliance obliges the United States to come to the defense of Japan in the event of an attack but does not require Japan to help defend the United States against aggression.

The Japanese government has maintained that the alliance benefits both countries. The treaty, under which around 54,000 U.S. troops are stationed in Japan, enables the United States to respond rapidly to contingencies in the Asia-Pacific region, including North Korea.

Trump said in the statement that the alliance is “rock-solid” and acknowledged that it has been “essential to peace, security, and prosperity” for both countries and the region over the past six decades.

On Sunday, Japan and the United States marked the 60th anniversary of the signing of their security treaty, an alliance they call the “cornerstone” of peace and stability in the region.

Prime Minister Shinzo Abe and members of his Cabinet participated in a commemorative reception in Tokyo on Sunday afternoon, which was also attended by Joseph Young, charge d’affaires of the U.S. Embassy in Japan, and Lt. Gen. Kevin B. Schneider, commander of U.S. forces stationed in the country.

“Today, more than ever, the Japan-U.S. security treaty is a pillar that is indestructible, a pillar immovable, safeguarding peace in Asia, the Indo-Pacific, and in the world, while assuring prosperity therein,” Abe said in his speech.

Abe said the alliance should become even more robust to “safeguard freedom, liberty, democracy, human rights and the rule of law, one that sustains the whole world, 60 years and 100 years down the road.”

The treaty is the basis for the U.S. military’s stationing of troops in Japan. Along with U.S. bases, Japan also hosts the Ronald Reagan, the only American aircraft carrier to be home-ported abroad.

In June last year, Trump complained that under the treaty, even if the United States were attacked, Japan would not be required to help and could “watch it on a Sony television.”

The alliance has faced domestic criticism, too. Some point to Japan’s position under the U.S. “nuclear umbrella” as being at odds with the country’s efforts to abolish nuclear weapons as the world’s only target of an atomic bombing.

There is also persistent local opposition against hosting U.S. forces in Okinawa, which is home to about 70 percent of the total area of land exclusively used by U.S. military facilities in Japan, amid repeated accidents and cases of assault and rape by American troops.

Still, the foreign and defense ministers of Japan and the United States said Friday in a joint statement that the alliance “has played and will continue to play an integral role in ensuring the peace and security of our two countries, while realizing our shared vision of a free and open Indo-Pacific.”

“Our alliance is stronger, broader, and more essential today than ever,” Foreign Minister Toshimitsu Motegi and Defense Minister Taro Kono said along with their U.S. counterparts, Secretary of State Mike Pompeo and Defense Secretary Mark Esper.

Signed in 1960 by the governments of then-Prime Minister Nobusuke Kishi — Abe’s grandfather — and then-President Dwight Eisenhower, the Treaty of Mutual Cooperation and Security between Japan and the United States replaced a 1951 agreement that helped form the basis of relations between the countries following the end of World War II.

The revised treaty removed a clause in the earlier version that allowed the United States to intervene to quell insurgencies within Japan, and made explicit Washington’s obligation to defend Japan from an armed attack.

Abe has worked to boost Japan’s role in the alliance, in 2014 removing an outright ban on exporting weapons and reinterpreting the pacifist Constitution to allow the Self-Defense Forces to protect allies in certain situations under collective self-defense.