NASA scientists have identified the galaxy's smallest-ever black hole, which is just the size of a city, but could still stretch a person into a "strand of spaghetti".

The miniature black hole, named J1650, has less than four times the mass of our sun, but researchers warn its pull - likely stronger than that of bigger black holes found at the centre of galaxies - could still play havoc.

"This black hole is really pushing the limits," said Nikolai Shaposhnikov of NASA's Goddard Space Flight Center.

"For many years astronomers have wanted to know the smallest possible size of a black hole, and this little guy is a big step toward answering that question."

He said that if someone ventured too close to J1650, which is located in the southern constellation Ara, in the Milky Way galaxy, its gravity would "stretch your body into a strand of spaghetti".

Like other black holes, it was formed by a star that ran out of fuel and shut down, collapsing due to its own gravity.

Neutron star

Mr Shaposhnikov and his Goddard colleague Lev Titarchuk used NASA's Rossi X-ray Timing Explorer satellite and a new method to estimate the size of the black hole.

Their method measures the oscillation of hot gas piling up near the black hole as it sucks in matter - it revealed the new black hole has a mass of 3.8 Suns and would be about 24 km across.

"This makes the black hole one of the smallest objects ever discovered outside our solar system," Mr Shaposhnikov said.

The smallest black hole previously identified was GRO 1655-40, with a mass of about 6.3 Suns.

"Amazingly, equations from Albert Einstein predict that a black hole with 3.8 times the mass of our Sun would be only 15 miles (24 km) across - the size of a city," NASA said in a statement.

A collapsing star that was much smaller than J1650 would likely form a neutron star and not a black hole, the researchers said.