"Human rights, son of a b----. Policemen and soldiers have died on me," he told reporters at a press conference. "Human rights — you go there and you might have a bomb dropped on your head."
Duterte has a history of salty language. He faced backlash for insulting then-President Obama on multiple occasions, calling him the "son of a w----" and telling him to "go to hell."
In the Philippines, the State Department has mostly stressed its concerns about the arrival of foreign ISIS fighters from the Middle East and about the longtime U.S. ally's drift closer to China and Russia.
The Duterte government has been struggling against an ISIS affiliate that seized control of the city Marawi on the southern island of Mindanao in May.
The U.S. has also provided "some training and some guidance in terms of how to deal with an enemy that fights in ways that is not like what most people have ever had to deal with," he added.
Last month Gen. Paul Selva, the vice chairman of the Joint Chiefs of Staff, brought up the possibility of an operation assisting the Philippines in its fight against the ISIS affiliate, leading to speculation that the U.S. could be expanding its involvement with Duterte's government.
"In every case where we see the resurgence of terror networks, particularly in the fragile areas of the southern Philippines, I think it is worth considering whether or not we reinstate a named operation not only to provide for the resources that are required but to give the Pacific Command commander and the field commanders in the Philippines the kinds of authorities they need to work with indigenous Philippine forces to actually help them be successful in that battle space," Selva told Congress in testimony last month.
Increased military cooperation with the Philippines could be seen by the U.S. as a buffer against Russia's and China's influence in the region after Duterte challenged his country's decades-old alliance with the U.S. After touring two Russian warships Friday, Duterte said, "We welcome our Russian friends. Any time you want to dock here for anything — for play, for replenish supplies or maybe our ally to protect us."
Defending the Trump administration's ties to Duterte, Tillerson said U.S. support for the Philippine fight against ISIS is not equivalent to support for the country's war on drugs.
"I see no conflict, no conflict at all in our helping them with that situation and our views of the human rights concerns we have with respect to how they carry out their counternarcotics activities," Tillerson told reporters Monday.
But Duterte made a direct connection between the two after his meeting with the U.S. delegation.
"The war in Marawi — what caused it but drugs?" he said.
Before the meeting, Tillerson and Duterte smiled for photographers and shook hands at the presidential palace in Manila, ignoring American reporters' shouted questions about human rights and the issues they would discuss.
As members of the press were ushered out, Duterte said to Tillerson, "You come at a time when the world is not so good, especially on the Korean Peninsula."
The U.S. has been seeking more pressure on North Korea from its Asian neighbors to help bring Pyongyang to the negotiating table about its nuclear and missile programs.