June 27 (UPI) -- President Donald Trump on Friday signed a peace deal between Rwanda and the Democratic Republic of Congo that ends 30 years of conflict in central Africa that the U.S. leader described as "one of the worst wars anyone's ever seen." Congo Foreign Minister Therese Kayikwamba Wagner and Rwandan Foreign Minister Olivier Nduhungirehe appeared at a signing ceremony in the White House's Oval Office. U.S. Secretary of State Marco Rubio and Qatar began negotiations with the two foreign ministers in April. The agreement was announced by the State Department on June 18. It is officially named the Washington Accords. "At least 6 million people were killed during that period of time," Trump said. "It's incredible. And somebody said that was actually, it's the biggest war on the planet since World War II. It's a shame but we're going to bring it to an end." The treaty ends the conflict and provides access to critical minerals for the U.S. "This is a wonderful day," he added. "Hopefully, there can be a lot of healing." Rubio, calling Trump a "president of peace," said: "This was not easy. And there's still work to be done, obviously, in terms of implementation, but we're very honored you're both here, and it's been an honor to work with both of you. " The two leaders thanked Trump, with Nduhungirehe describing the treaty as a "remarkable milestone." But they urged the United States ensure the peace agreement remains in place. "There have been many mediations in the past but none of them succeed," Nduhungirehe said. "We believe that it is because of your leadership and steadfast commitment the treaty was made possible." Wagner said: "This moment has been long in coming. It will not erase the pain, but it can begin to restore what conflict has robbed many women, men and children of: safety, dignity and a sense of future." Democratic senators also urged the treaty to be enforced and humanitarian assistance provided. "While signing an agreement is important, implementation will be essential, and we urge both parties and all international partners to ensure its enforcement," Sens. Chris Coons of Delaware, Corey Booker of New Jersey and Jeanne Shaheen of New Hampshire said in a statement. "We are keenly watching how today's agreement shapes the future of eastern DRC. This is where the hard work begins, and following through on each component of the deal will be essential to its success." In the accord, both sides agreed to recognize and respect each other's territorial borders, committed to not supporting any armed groups and to establish a joint security mechanism to target militias. And they plan to expand trade and investment opportunities. Around 7 million people have displaced in Congo, which has a population of 106 million. Rwanda's population is 14 million. They both gained independence from Belgium in the early 1960s. In January, M-23 rebels were aided by Rwandan forces in escalating the conflict, according to a United Nations expert panel. They seized the strategic cities of Goma and Bukavu. M23 first emerged in 2012. The region has been reeling from one of the world's worst humanitarian crises. The rebels overwhelmed government forces, killing U.N. peacekeepers, fired on U.N. humanitarian facilities and sent people fleeing from displacement camps. The United Nations has called it "one of the most protracted, complex, serious humanitarian crises on Earth." Congo wants the U.S. to provide security support needed to fight the M23 rebels. They want them to withdraw from Goma and Bukavu, and from the entire region where Rwanda is estimated to have up to 4,000 troops. The United States imposed sanctions on key Rwandan officials involved in the conflict. The European Union cut military aid to Rwanda.