Donald Trump has said he does not think North Korea is “ready to negotiate” after it launched suspected short-range missiles, in the second such weapons test in five days.

“We’re looking at it very seriously right now,” Trump told reporters in the White House. “Nobody’s happy about it.”

He said the relationship with Kim Jong-un’s government would continue, but added: “I don’t think they’re ready to negotiate.” US officials had previously expressed optimism that Trump would meet Kim for a third summit, aimed at persuading North Korea to dismantle its nuclear programme in return for sanctions relief.

Administration officials have pointed to an extended hiatus in nuclear and missile tests since late 2017 as the most concrete achievement of Trump’s personal diplomacy with Kim, which has so far led to two summit meetings, in Singapore and Hanoi, but no significant steps towards North Korean disarmament or any easing of sanctions.

The state-run news agency published pictures of Kim Jong-un supervising a military drill on 10 May. Photograph: KCNA/Reuters

In another development likely to dent hopes of a resumption of talks, the US justice department said it had taken custody of a North Korean cargo ship it alleged had been involving in sanctions-busting by smuggling coal from Russia in defiance of a UN embargo.

The ship, the Wise Honest, was first seized by Indonesian authorities in April 2018, after it was spotted sailing erratically, veering out of shipping lanes, with its transponder turned off. It was found to be carrying coal that was allegedly intended for a ship-to-ship transfer in international waters.

Geoffrey Berman, a US attorney, told reporters: “Today’s civil action is the first ever seizure of a North Korean cargo vessel for violating international sanctions.

“This scheme not only allowed North Korea to evade sanctions, but the Wise Honest was also used to import heavy machinery to North Korea, helping expand North Korea’s capabilities and continuing the cycle of sanctions evasion.

“With this seizure, we have significantly disrupted that cycle. We are willing and able to deploy the full array of law enforcement tools to detect, deter and prosecute North Korea’s deceptive attempts to evade sanctions.”

Although it was first seized before the Trump-Kim summits began, the announcement that the Wise Honest had been confiscated by the US and taken out of service is likely to sour the relationship. In the past, North Korea has responded defiantly to efforts to impose pressure on the regime, declaring them to be “acts of war”.

According to South Korea’s joint chiefs of staff, the projectiles in the latest North Korean weapons test flew 260 miles (420km) and 167 miles respectively. They said they were working with the US to determine more details, such as the type of weapon that was fired.

On Thursday night in Washington, the Pentagon said North Korea had launched multiple ballistic missiles that flew 300km (186 miles) into the ocean.

South Korea’s military said earlier that at least one projectile was launched from the Sino-ri area of North Pyongan province, an area known to have one of North Korea’s oldest missile bases and where a brigade operates mid-range Rodong missiles.

It later said the launch was from the nearby town of Kusong, where North Korea conducted the first successful flight test of its Hwasong-12 intermediate range missile in May 2017.

Kusong is also home to missile test facilities that were critical to the development of North Korea’s solid-fuel Pukguksong-2, which was successfully flight-tested for the first time in February 2017, in the North’s first missile test after Trump took office.

The launches came as the US special representative for North Korea, Stephen Biegun, was visiting South Korea, and hours after Pyongyang described the firing of rocket artillery and an apparent short-range ballistic missile on Saturday as a regular and defensive military exercise. North Korea also ridiculed the South for criticising those launches.

South Korea’s presidential Blue House said in a statement that the North Korean launches on Thursday were “very concerning” and detrimental to efforts to improve inter-Korean ties and ease military tensions on the peninsula.

Some analysts have said that if North Korea resumes testing the kind of longer-range, banned ballistic weapons it fired in unusually large numbers in 2017, when many feared a US-North Korea standoff could end in war, it may signal Pyongyang was turning away from diplomacy.

The tensions were followed by a surprising diplomatic outreach by Kim in 2018, when he attended summits with the South Korean and Chinese presidents, and with Trump. But Pyongyang has not received the relief it desires from punitive sanctions imposed over its nuclear and missile programmes.

A summit in February between Trump and Kim ended in failure, with the US saying North Korea was not offering to take enough disarmament steps in return for widespread sanctions relief.

Before the launches on Thursday, senior defence officials from South Korea, the US and Japan met in Seoul to discuss North Korea’s earlier launches and other security issues. Details from the meeting were not immediately announced.

Experts who analysed photos from North Korean state media say the North tested a new solid-fuel missile on Saturday that appears to be modelled on Russia’s Iskander short-range ballistic missile system.

North Korea is trying to press South Korea into turning away from the US and supporting Pyongyang’s position more strongly, said Cha Du-hyeogn, a visiting scholar at the Asan Institute for Policy Studies in Seoul. After the collapse of the Trump-Kim meeting, North Korea demanded the South proceed with joint economic projects that have been held back by US-led sanctions against the North.

By firing weapons that directly threaten South Korea but not the US mainland or its Pacific territories, North Korea also appears to be testing how far Washington will tolerate its bellicosity without allowing the nuclear negotiations to collapse, Cha said.