"It is creating some stress within North Korea's economy," Tillerson said, pointing to US and Chinese intelligence.
"There are clear signs and the Chinese side has shared with us some of the signs that they're seeing. We see certain signs with our own human intel and other sources we have," Tillerson said, following a day of meetings between President Donald Trump, Chinese President Xi Jinping and top officials from both governments.
The comments mark the first time the US has indicated that the latest round of UN sanctions are beginning to affect North Korea's economy.
China signed onto a US-drafted UN Security Council resolution in September that imposed fresh sanctions against the North Korean regime, but Trump's visit in Beijing this week was intended to press China to increase its pressure on Pyongyang.
Tillerson emerged from the day of meetings with Chinese officials to state that "there is no disagreement on North Korea," though he conceded that there is some divergence on how to deal with the issue.
"There is no space between both of our objectives," Tillerson said, adding the views on how to accomplish their shared goal is 'what we spend a lot of time on."
Tillerson said Trump specifically takes the view that China is a "very powerful neighbor" of North Korea and can solve the issue from that position.
Trump and Xi had a "very, very detailed" exchange on North Korea during Trump's state visit to Beijing, Tillerson said.
Asked whether China was complying with UN resolutions on North Korea, Tillerson said the country was in compliance.
Trump and Xi had a "frank, open and productive" Thursday that included talks on human rights and disputes in the South China Sea, according to Tillerson.
On maritime arguments, Tillerson said the US view hadn't changed that "claimants should stop construction and militarization" of disputed islands.
And he said the men discussed ways to "prevent mishaps, misunderstands and miscalculations."