Global space agencies are closely monitoring a building-size space rock that has a tiny chance of hitting Earth seven years from now.

Nasa and the European Space Agency (ESA) estimate that the asteroid, called 2024 YR4, is around 40 to 100 metres wide.

The ESA said it has an almost 99 percent chance of safely passing Earth on December 22, 2032, but a possible impact could not be entirely ruled out.

Astronomy expert, Professor Quentin Parker from the University of Hong Kong, said if the asteroid hits Earth, it would cause a devastating impact.

"It's like a 10 megaton nuclear explosion on that city. But the likelihood is, given that two-thirds of the Earth's surface is water, it'll go into the ocean... creating a small tsunami, which actually could be quite dangerous to landfall around where that occurred," he told RTHK.

"If it hits in a desert, or in an unpopulated region, it'll devastate the surrounding region up to maybe several tens of kilometres in terms of graded damage from the epicentre to the edge where the damage is completely catastrophic at the impact site, then drops off as you get further away."

But he said there are still many unknowns and scientists could make more accurate estimations when the asteroid comes closer to Earth again in 2028.

"It's hard to study because it has a very elliptical orbit. It goes very far away and comes close. You can only study when it comes close. When it comes close again in 2028, we'll be able to refine the orbit again even further," Parker said.

"There's nothing we can do in terms of sending out a rocket at the moment to deflect it."

The asteroid now stands at the top of the ESA's asteroid risk list and Nasa's automated Sentry risk list, which include any known asteroids with a non-zero probability of colliding with Earth.