Japan is to boost its ability to counter the “serious and imminent” threat posed by North Korea’s ballistic missiles with the deployment of ground-based interceptors bought from the US.

The defence ministry will buy two Aegis Ashore land-based systems to add to its SM-3 guided missiles launched by Aegis-equipped destroyers and Patriot Advanced Capability-3 missiles.

The cabinet said it had taken Tuesday’s decision to approve the expensive hardware in response to growing threats.

“North Korea’s nuclear and missile development has become a greater and more imminent threat for Japan’s national security, and we need to drastically improve our ballistic missile defence capability to protect Japan continuously and sustainably,” a cabinet statement said.

North Korea test-launched two ballistic missiles over Japanese territory this year, sparking text alerts warning millions in the north of the country to take cover.

“North Korea’s nuclear missile development poses a new level of threat to Japan and as we have done in the past we will ensure that we are able to defend ourselves with a drastic improvement in ballistic missile defence,” Itsunori Onodera, the defence minister, said.

Last month, North Korea tested an intercontinental ballistic missile, the Hwasong-15, it claims can strike US cities, although experts have questioned whether a missile carrying a nuclear payload, and flying on a standard trajectory, would be capable of covering that distance.

Q&A What do we know about the Hwasong-15 missile? Show The Hwasong-15 missile fired on 29 November flew on a steep trajectory for 50 minutes, reaching an altitude of 2,800 miles (4,500 km) and distance of 620 miles, according to North Korea. The US-based Union of Concerned Scientists said that if the numbers were correct, then the missile would have a range of 8,080 miles on a standard trajectory. That figure suggests that all of the US could theoretically be within range. The missile appears to be an advanced version of the Hwasong-15 ICBM tested in July by North Korea, which claims this version makes it a "complete" nuclear state. Pyongyang has not, however, proved it has the capability to marry a miniaturised nuclear warhead with a long-range missile and send it at a trajectory that would hit US cities. It also remains unclear whether the North Koreans have perfected a re-entry vehicle capable of protecting a nuclear warhead during its descent. Photograph: Kim Hong-Ji/X90173

Defence ministry officials in Tokyo refused to discuss details of the Aegis Ashore deployment until the official release of the 2018 defence budget, which is expected to rise to a record $46bn.

Reports said Aegis Ashore’s deployment at two bases in northern and south-western Japan could cost a total of 200bn yen ($1.8bn). The systems will be capable of covering the entire country and are expected to be operational by 2023.

“We believe this new system will significantly boost our abilities to deal with surprise attacks and saturation attacks,” said a defence ministry official quoted by Kyodo news.

There is concern over the progress Pyongyang appears to be making in perfecting its nuclear capability and this has fuelled calls for Japan to be able to strike pre-emptively at North Korean missile sites.

Onodera has said Japan will acquire medium-range cruise missiles that could be launched at North Korean targets from fighter planes if an attack appeared to be imminent.

It marks a significant shift from Japan’s strictly defensive postwar posture, on which it depended on its US ally to carry out pre-emptive strikes at enemy targets.

Critics argue that a first-strike capability violates Japan’s postwar renunciation of force as a means of settling international disputes, as set out in its US-authored constitution.