All three nations are permanent members of the U.N. Security Council which this week approved new sanctions to punish Pyongyang's September 3 atomic test, its largest to date.
The move announced by European Union headquarters Thursday follows the Security Council resolution of August 5, which bans North Korea from exporting coal, iron, lead and seafood products estimated to be worth over $1 billion.
The unanimous adoption of the resolution was still significant because it sent a message to the North Korean regime that the worldwide community views its weapons programs seriously, he said.
The U.N. resolution also bans countries from giving additional work permits to North Korean laborers, another source of money for Pyongyang.
"I think it's clear that with respect to oil and a complete embargo on oil from the U.N. Security Council, that's going to be very hard", Tillerson added, according to an official transcript.
On Thursday, Tillerson singled out China as the supplier of most of North Korea's oil and Russian Federation as the largest employer of North Korean laborers working overseas.More news: Graham-Cassidy Bill Introduced, Immediately Blasted By Dems
U.S. Secretary of State Rex Tillerson delivers remarks to the employees at the State Department in Washington, U.S., May 3, 2017.
'They also discussed North Korea and its continuing destabilising activities. The worldwide community adopted new measures on 11 September which put in place the most stringent UN sanctions regime placed on any nation in the 21st century.
On the Iran nuclear agreement, which is viewed skeptically by US President Donald Trump, May was "underlining its importance in preventing Iran from procuring nuclear weapons", her spokesman said.
Last month, North Korea fired another ballistic missile from a similar area near the capital that flew over Hokkaido before splashing down into the ocean, as well.
Later, representatives from the U.N., Italy, Egypt and the United Arab Emirates joined for discussions on the upheaval in Libya and a U.N. -backed mediation effort to stabilize the North African nation.