Pyongyang could have more nuclear arsenal than previously thought, believe Chinese experts and suspect that North Korea may already have around 20 nuclear warheads and a uranium enrichment capacity of doubling that number by next year.

This exceeds the US Congressional estimate of the country possessing around 10 to 16 nuclear weapons.

The Wall Street Journal has reported that the Chinese estimate, which was relayed to US nuclear specialists in a closed-door meeting in February, is close to the higher side of a range estimated in a report published in February by the US-Korea Institute at the Johns Hopkins School of Advanced International Studies.

That report stated that the country had 10 nuclear weapons at the end of 2014, while this could double by 2020. The report also assumes that Pyongyang would struggle to find resources by the end of 2020 and would halt producing the weapons thereon.

It also estimated that in a worst case scenario, North Korea could possess around 100 atomic weapons by 2020.

According to the report, Pyongyang has succeeded in miniaturising nuclear warheads, and its missiles can reach neighbouring South Korea and Japan, while it is developing a longer range model capable of targeting the US.

The latest Chinese estimate reflects growing concern in Beijing about the nuclear ambitions of its neighbour and ally.

The capacity to produce centrifuges needed to enrich uranium and how much of the fissile material would go into making each bomb are some of the factors experts are studying.

And the journal suggests that the country has robust retaliatory capabilities to pose a threat to the United States.

Meanwhile, Siegfried Hecker, a leading expert on North Korea's nuclear programme, who attended the February meeting, has warned that a sizeable North Korean stockpile would make it difficult to persuade Pyongyang to denuclearise.

North Korea carried out nuclear tests in 2006, 2009 and 2013 and has an active ballistic missile development programme.

Pyongyang often launches missiles to protest joint military exercises by the US and South Korean forces in the Korean Peninsula.

The regime had recently declared it has the capability of launching a nuclear missile at "anytime" if attacked. While experts acknowledge the nation has the ability to build nuclear weapons, it remains unclear whether it possesses the technology to miniaturise warheads and mount them on ballistic missiles.

In a white paper published in January, the South Korean defence ministry said the North had already taken its miniaturisation technology to a "significant" level.