Ri was speaking Saturday at the United Nations General Assembly. He called on the United States to follow through on promises made during a summit in Singapore between the rivals' leaders.
His comments come as US. Secretary of State Mike Pompeo seems to be on the verge of restarting deadlocked nuclear diplomacy more than three months after Singapore with North Korea's Kim Jong Un.
Ri says it's a "pipe dream" that continued sanctions and U.S. objection to a declaration ending the Korean War will ever bring the North to its knees.
Washington is wary of agreeing to the declaration without Pyongyang first making significant disarmament moves.
Both Kim and U.S. President Donald Trump want a second summit. But there is widespread skepticism that Pyongyang is serious about renouncing an arsenal that the country likely sees as the only way to guarantee its safety.
Pompeo is planning to visit Pyongyang next month to prepare for a second Kim-Trump summit.
Syria's foreign minister is demanding the immediate withdrawal of all foreign troops from Syria who are there in the country "illegally."
He cites U.S., French and Turkish troops who are in Syria without invitation from the Syrian government.
He says the Syrian government considers them "occupation forces and will be dealt with accordingly."
The United States has around 2,000 troops in northern Syria, working with local forces against Islamic State militants in the country.
Syrian foreign minister Walid Muallem says a committee tasked with drafting a new constitution will not accept outside dictates.
He says anything seen as intervention in Syria's internal affairs is rejected.
He spoke on Saturday, a few days after the United States and six other nations called for the United Nations to convene a committee to begin drafting a new constitution for Syria and promote a political transition in the war-ravaged country.
"Any conditions or pre-conclusions on the work of the committee will not be accepted," Muallem says.
The U.N.-led effort to bring Syria's warring factions together to draft a new constitution under which elections would be held has been stalled for years.
Syria's foreign minister is telling world leaders that his country's "battle against terrorism is almost over."
Walid al-Moallem also vowed Saturday at the U.N. General Assembly that the Syrian government will free the country from all "illegitimate" foreign troops. And he called on all refugees to return home, saying that is a priority for Damascus.
Syrian government forces, backed by Russia and Iran, have retaken most of the territory rebels seized during the war that has killed over 400,000 people and driven millions from their homes.
The deadly Indonesian tsunami is on the minds of some world leaders as they address the U.N. General Assembly less than a day later.
Indian foreign minister Sushma Swaraj expressed condolences to Indonesia during her speech Saturday. She promised that "India will cooperate in helping during this tragic time."
The tsunami swept away buildings and killed at least several hundred people on the central Indonesian island of Sulawesi. Disaster agency spokesman Sutopo Purwo Nugroho said 384 people were killed in the hard-hit city of Palu alone.
The nearby city of Donggala and the town of Mamuju were also ravaged by the magnitude 7.5 earthquake and tsunami. But aid had not reached those communities, due to damaged roads and disrupted telecommunications.
Two of the planet's most troubled hot spots are sending envoys to have their say before the world.
North Korea and Syria are places of international concern that spill trouble beyond national borders, and are to appear before the U.N. General Assembly on Saturday, at a time when both countries could be on the cusp of significant developments.
U.S. President Donald Trump and his secretary of state, Mike Pompeo, are trying to regain momentum more than three months after the Singapore summit with North Korea's Kim Jong Un. They are pushing for a second meeting in their quest to get Pyongyang to renounce its nuclear ambitions.
Syria could be on the verge of emerging from seven years of bloody conflict that included the use of chemical weapons.
Syria's foreign minister will take the podium with his government firmly in control of most of the territory the opposition held for many of the seven years of the conflict.