WASHINGTON - Pentagon officials are assuring members of Congress that a massive explosion planned at the Nevada Test Site won't lead to new nuclear weapons development.

But Utah Rep. Jim Matheson is skeptical.

"We're real anxious to find out what's going on here, because I'm real suspicious," said Matheson, a Democrat.

The Pentagon has said it plans to detonate 700 tons of explosives above a buried tunnel at the Nevada Test Site in June.

The Divine Strake test, as it is called, is designed to help hone computer models that will help war planners select the smallest nuclear weapon that would be needed to destroy a buried, hardened target.

"I think this is a precursor for the development of nuclear weapons. I've had that suspicion about this administration all along," Matheson said. "I want to get answers to this before they have the test in June."

Sen. Harry Reid and Rep. Shelly Berkley, Democrats from Nevada, also voiced concern over the test and met with James Tegnelia, director of the Defense Threat Reduction Agency, for briefings Thursday. Both said they were confident the test would be safe.

In the private meetings, Tegnelia insisted that the Divine Strake test is not intended as a step toward renewed nuclear testing, Reid and Berkley said.

"I do not support the resumption of nuclear testing, but this test uses only commercial explosives," Reid said in a statement. "It seems to be well-planned, and all the necessary environmental tests and safety precautions are being done. At this point, there is no reason for the test not to go forward."

Berkley said she grilled Tegnelia on the specifics of the test and was reassured it will be safe.

"I will not seek to block this mammoth explosion, which will create a plume over the Nevada Test Site, but I will keep a close eye on how the public is being prepared and I will demand full answers to any new concerns that may arise," Berkley said in a statement.

The Divine Strake test would use ammonium nitrate and fuel oil - a common explosive combination and the same used in the Oklahoma City bombing, only 280 times larger.

It is expected to throw a plume of dust as high as 10,000 feet into the air. Computers on the ground will measure the shockwaves and damage to the tunnel so models can be made, according to Pentagon budget documents, "to improve the warfighter's confidence in selecting the smallest proper nuclear yield necessary to destroy underground facilities while minimizing collateral damage."

The blast would be fifty times larger than the largest existing conventional weapon, but many times smaller than the smallest nuclear weapon in the U.S. stockpile. Similar tests have been conducted in the past at White Sands Missile Range in New Mexico.

Since 1994, there had been a ban on development of mini-nukes, or tactical weapons that could be used in the battlefield. The Bush administration successfully pressed Congress to repeal that ban in 2003.

Tegnelia told Reid and Berkley there is no radioactive material in the test or left over in the soil from prior tests at the range. Nearby communities might feel tremors from the blast, but are not at risk, he said.

Tegnelia assured the Nevada members that the Defense Threat Reduction Agency would be bullish about providing the public with information on the blast and environmental studies. The news media will also be allowed on the site on the day of the test.

Tegnelia also apologized for earlier statements referring to a mushroom cloud over the Nevada Test Site for the first time since nuclear testing ended.