(CNN) Radiation levels across parts of the Marshall Islands in the Pacific Ocean, where the United States tested nuclear bombs during the Cold War, are higher than areas contaminated by the Chernobyl and Fukushima nuclear disasters, new research suggests .

From 1946 to 1958, the US government conducted 67 nuclear tests on several small islands -- called atolls -- in the Marshall Islands

The US government relocated entire populations and exposed others to cancer and disease-causing radiation.

More than 60 years later, researchers at Columbia University say radiation on four of these atolls remains alarmingly high -- in some areas ten to 1,000 times higher than radioactive areas near the Chernobyl powerplant, which exploded in 1986, and Fukushima , where an earthquake and tsunami caused a nuclear disaster in 2011.

Analyzing soil samples, researchers found concentrations of americium-241, cesium-137, plutonium-238, and plutonium-239,240 on 11 islands across the four northern atolls.

The population of the Marshall Islands is relatively small, with just over 75,000 people living on the chains as of July 2018. It is a combination of islands and atolls, which are usually circular islands ringing a wide lagoon or coral reef.

Some of the atolls and islands have just a few hundred people on them. Enewtak Atoll was home to just 664 people in the 2011 census.

Enewtak was one of two atolls, along with Bikini, which were described by researchers as "ground zero" for the nuclear tests.

Satellite image of Castle Bravo Crater in Bikini Atoll, part of the Marshall Islands in the Pacific Ocean on March 9, 2017.

While a fraction of the 1,054 total nuclear tests carried out by the US from 1946 to 1992 took place on the Marshall Islands, the coral atolls withstood more than half the total energy yielded from all US nuclear tests during that time, the researchers said.

Bikini was the site of the US's largest hydrogen bomb test known as Castle Bravo in 1954 -- the blast was 1,000 times as powerful as those dropped on Japan during World War II.

Bikini Island was found to have the highest levels of radiation of areas studied, with the report's authors recommending that Bikini remains uninhabited, owing to its high levels of radiation.

Residents of Bikini atoll were forcibly relocated in 1946 and were shipped around to several different islands due to unsustainable food and water sources. Researchers said some Bikinians briefly returned to Bikini Island in the late 1960s after the US government declared the island safe for resettlement, but soon left due to high levels of radiation exposure.

Elsewhere, researchers said that the presence of radioactive isotopes on Runit Island in Enewtak atoll was "a real concern" and recommended that people should be warned "against any use of the island."

Following US occupation during World War II, the islands became a US territory until the late 70s. It is now an independent nation but has a Compact of Free Association with the US.

According to the Environmental Protection Agency, exposure to very high levels of radiation, such as being close to an atomic blast, can cause skin burns and acute radiation syndrome, known as radiation sickness. It can also result in long-term health effects such as cancer and cardiovascular disease.

The report also raises concerns that wash-off of existing isotopes into the ocean would be exacerbated by rising sea levels and risked contaminating the island's lagoon and the surrounding ocean.

Several hundred people currently live on Enewetak Island in the south of the atoll after a massive radioactive cleanup in 1980.

The other two atolls -- Rongelap and Utirik -- were significantly affected by the radiative fallout from the Bravo test.

In Rongelap atoll, researchers found that northern Naen island had the highest levels of external gamma radiation of all islands examined in the study -- well above the legal exposure limit agreed between the Republic of the Marshall Islands and the US. Soil samples on Naen were also found to have high concentrations of radioactive isotopes.

An untested theory for the high levels of radiation on Naen could be that the island may have been used as a dumping ground for some of the waste from the cleanup on Rongelap, researchers suggest.

Publishing their findings in the Proceedings of the National Academy of Sciences this week, researchers said they aimed to present a view of the current "radiological conditions" on the impacted atolls, focusing on uninhabited islands that are often used as food sources.