SEOUL, South Korea — The Obama administration sought Wednesday to add pressure on North Korea by announcing expanded and strengthened sanctions against the communist-led government and its nuclear weapons program. In the past similar tactics have been undercut by North Korea's knack for finding loopholes and escape hatches.

The fresh round of sanctions was announced by Secretary of State Hillary Rodham Clinton during a visit to Seoul, South Korea, but officials in Washington said the moves will not be ready to be put in place for another two weeks.

The State Department announced that Robert Einhorn, charged with overseeing the administration's implementation of sanctions against North Korea and Iran, will travel to Asia in early August, after sanction details are final, to rally support among U.S. friends and allies.

China, whose role in enforcing North Korea sanctions is considered vital, is a likely stop on Einhorn's tour. State Department spokesman P.J. Crowley declined to discuss the diplomat's itinerary.

Clinton is expected to discuss the sanctions with Chinese officials during an Asia security conference in Vietnam this week. North Korean Foreign Minister Pak Ui Chun is expected to attend the meetings, although he and Clinton are not expected to engage in any face-to-face talks.

In Seoul, Clinton said the U.S. moves will target arms deals and other clandestine trade used to finance the North Korea's nuclear activities, and imports of luxury items for the ruling elite in Pyongyang.

"They are directed at the destabilizing, illicit and provocative policies pursued by that government," she said. Clinton was referring to North Korea's nuclear weapons program as well as its alleged large-scale counterfeiting, smuggling, drug trafficking and weapons technology exports.

The U.S. penalties are intended to isolate the hermit nation further and persuade its leaders to return to talks aimed at getting it to abandon atomic weapons. The United States also is trying to forestall future provocative acts like the March torpedoing of the Cheonan, a South Korean naval vessel, which killed 46 South Korean sailors. North Korea denies it sank the ship.

Bruce Bennett, a North Korea expert at the RAND Corp., a federally financed think-tank , said the new U.S. actions are a reflection of North Korea's ability to skirt aspects of earlier sanctions, including two rounds of extensive penalties passed by the U.N. Security Council.

"It's saying, 'We haven't been very effective in imposing the previously enacted sanctions,' and so this is an effort to try to strengthen the previous sanctions and make them work," Bennett said.

Crowley acknowledged that North Korea has found ways to evade earlier sanctions.

"North Korean entities are adapting to the existing actions that we have been taking," Crowley said. "We gain significant intelligence information all the time about what they're doing."

The spokesman said North Korea has created foreign front companies to help it get around U.N. sanctions.

"They look to see, you know, which countries have been effectively complying and enforcing, you know, U.N. Security Council resolutions. They look to see if there are seams and gaps in the international effort," he said.

The most recent round of U.N. sanctions was in June 2009. Those sought to deprive North Korea of financing and materiel for its weapons program and banned its arms exports, especially missiles. It did not ban normal trade, but called on international financial institutions to halt grants, aid or loans to the North except for humanitarian development and efforts to dismantle its nuclear programs.

Richard C. Bush III, director of Northeast Asia policy studies at the Brookings Institution, said in an interview that he believes the new U.S. sanctions are more than merely a response to the sinking of the Cheonan in March.

He believes they also are meant as a message to the next set of North Korean leaders, who are believed to be angling for positions of power in anticipation of the passing of the nation's current leader, Kim Jong Il.

Nicholas Szechenyi, a northeast Asia policy analyst at the Center for Strategic and International Studies in Washington, said the key to effective U.S. sanctions is how they are implemented.

"If the U.S. is doing this in isolation, doing this piecemeal, then I don't think they'll have much effect," he said. "But if there's a unified effort to not only announce these sanctions as an act of solidarity with our South Korean allies but also to apply some pressure on North Korea, then I think over time it might work."