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WASHINGTON (Reuters) - An increase in activity at a North Korean satellite launch site could indicate preparations for a new test or other activities, a U.S.-based monitoring group said on Saturday.

The 38 North group, run by Johns Hopkins University’s School of Advanced International Studies, said satellite images taken on Oct. 1 show increased activity at the Sohae Satellite Launching Station, including crates on the launch pad and vehicles near the fuel and oxidizer buildings.

But it added that since some of the structures on the launch pad are covered, “it is unclear whether this activity is related to launch preparations or other operations.”

North Korea conducted its fifth and biggest nuclear test on Sept. 9 and South Korea has said it believes the north is ready to conduct another nuclear test at any time. There has been speculation that Pyongyang could mark the Oct. 10 anniversary of the founding of its Workers’ Party with a sixth detonation.

North Korea has been testing nuclear weapons and ballistic missiles at an unprecedented rate this year under leader Kim Jong Un’s direction, including the launch of a satellite in February that was widely seen as a test of long-range ballistic missile technology.

The Sohae center is the North’s newly upgraded rocket station where the February satellite launch and other rocket tests have been conducted.

On Friday, 38 North said an increase in activity at North Korea’s Punggye-ri nuclear test site could signal preparations for a new test or a collection of data from its last one.

North Korea conducted its first nuclear test in 2006 and has since defied U.N. sanctions to press ahead with the development of the weapons and missiles to carry them, which it says it needs for defense.

In January, it conducted its fourth nuclear test and the fifth was carried out on the anniversary of the nation’s founding.