WASHINGTON, June 24 (Reuters) - The United States warned activists on Friday against plans to send a new aid flotilla to challenge Israel's blockade of the Gaza Strip, saying it would be irresponsible and dangerous.

Groups that seek to break Israel's maritime blockade of Gaza are taking irresponsible and provocative actions that risk the safety of their passengers, State Department spokesperson Victoria Nuland said in a statement.

Secretary of State Hillary Clinton said there were better ways of getting help into Gaza.

We don't think it's useful or productive or helpful to the people of Gaza, Clinton told reporters after a meeting with the visiting South Korean foreign minister.

We believe that a far better approach is to support the work that is being done through the United Nations ... to ensure that the people of Gaza get access to materials and humanitarian assistance in a safe and timely way.

Israel said on Wednesday it had warned the United Nations that a new aid flotilla -- which activists say could depart from European ports in coming days -- could result in dangerous consequences.

Israel has made clear it will prevent any new flotilla from reaching Gaza. A year ago, nine Turkish activists, including one with dual U.S.-Turkish nationality, were killed in an Israeli raid on a similar convoy.

The Israeli military came under fierce criticism for the May 2010 raid, which led to a severe deterioration of its ties with Turkey.

The United States, Israel's most important ally, has backed Israel's blockade of Gaza, which the Palestinian Islamist Hamas group seized from forces loyal to Western-backed Palestinian President Mahmoud Abbas in 2007.

Palestinians say the Israeli sea blockade is illegal and is helping strangle Gaza's underdeveloped economy.

Nuland said the United States remained concerned about conditions in Gaza, but that the situation there had improved significantly over the past year with a broader range of goods and materials available.

But she said that recent weapons seizures and periodic rocket and mortar attacks from Gaza against Israeli civilians illustrated the ongoing necessity for Israel to screen Gaza-bound cargo.

We underscore that delivering or attempting or conspiring to deliver material support or other resources to or for the benefit of a designated foreign terrorist organization, such as Hamas, could violate U.S. civil and criminal statutes and could lead to fines and incarceration, Nuland said.

Clinton said the people of Gaza were the victims of the decisions that have been made over the past years by Hamas, and that the United States hoped to see changes in the way the region is administered.

The contrast between the rising standard of living and economic opportunity and educational and health services in the West Bank, compared to Gaza, I think tell a very compelling story, she said.

(Editing by Jackie Frank and Peter Cooney)