The United States on Wednesday slapped sanctions on six North Korean ships, 16 individuals and nine companies that it said had facilitated Pyongyang's weapons programs in a continuing effort to further isolate the regime.

The sanctions are part of a strategy by the Trump administration to pressure North Korea into abandoning its nuclear weapons and long-range missiles.

Increasingly, the administration has been turning its attention to the smuggling going on despite a round of U.N. sanctions.

Secretary of State Rex Tillerson attended a conference in Vancouver last week in which diplomats from 20 nations discussed ways to intensify pressure on North Korea, particularly by stopping ship-to-ship transfers in open water. The United States is trying to build support for its campaign to get nations to blacklist ships involved in smuggling goods to North Korea from any port in the world and to conduct maritime interdiction.

The sanctions were announced shortly after Japan said one of its spy planes had photographed a North Korean tanker that was probably violating sanctions by transferring cargo from one ship to another.

According to Japan's Ministry of Foreign Affairs, photos taken Saturday show the Rye Song Gang 1 pulling alongside a Dominican-flagged ship, the Yuk Tang, in the middle of the night. Shortly after the sun rose on Saturday morning, the two ships sailed away from each other.

"The government strongly suspects the two engaged in offshore delivery which was banned by U.N. Security Council resolutions," the ministry said in a statement.

The sanctions by the Treasury Department's Office of Foreign Assets Control listed six ships that sail under the North Korean flag — the Goo Ryong; the cargo ships Hwa Song, Un Ryul and Ever Glory; the tanker Kum Un San; and the cargo ship Ul Ji Bong 6, which loaded coal at a North Korean port and delivered it to Russia in September.

Under the sanctions, the vessels are blocked, which imposes a ban on transfers or dealings of any kind. Americans are prohibited from dealing with the companies that own them, and international banks that do any kind of transactions with them are subject to U.S. sanctions.

Several of the 16 individuals hit with sanctions work in China near the North Korean border at the Korea Ryonbong General Corp., which the Treasury Department already has imposed sanctions on. According to the Treasury Department, Ryonbong supports North Korea's military, helping it acquire chemicals, drilling equipment, hoses, metal and machines.

Some work for Ryonbong in Russia or Georgia, including one said to arrange jobs overseas for North Korean laborers whose salaries largely go to the North Korean government.

Other sanctioned individuals work for banks in China and Russia that are linked to a North Korean bank.

The sanctioned companies are from China and North Korea — Beijing Chengxing Trading Co.; Dandong Jinxiang Trade Co.; Hana Electronics, which is one of North Korea's few electronics manufacturers; and the owners of the six North Korean ships: Gooryong Shipping, Hwasong Shipping, Korea Kumunsan Shipping, Korea Marine and Industrial Trading, and CK International.

According to the Treasury Department, Chengxing Trading Co. has sold two tons of high purity metals to a subsidiary of Ryonbong. The Dandong Jinxiang Trade Co. exported more than $500 million of used computers and other goods to North Korea.

"Pursuant to UN Security Council Resolutions, the U.S. government is targeting illicit actors in China, Russia, and elsewhere who are working on behalf of North Korean financial networks, and calling for their expulsion from the territories where they reside," Treasury Secretary Steven Mnuchin said in a statement. "We are sanctioning additional oil, shipping, and trading companies that continue to provide a lifeline to North Korea to fuel this regime's nuclear ambitions and destabilizing activities."