Trade Minister David Parker has been unable to secure an exemption from US steel and aluminium import taxes but says his officials will continue lobbying New Zealand's case.

But Parker was not sounding optimistic about their chances of winning US trade officials over, telling the Herald "the longer it goes on without being resolved, the less confident we would be" of securing an exemption.

Parker spent the better part of this week in Washington DC "advancing New Zealand's interests in reform of the World Trade Organisation (WTO)" as well as advocating for an exemption to US tariffs on steel and aluminium.

The tariffs were imposed in March this year – soon after, Prime Minister Jacinda Ardern wrote a letter to US President Donald Trump pleading New Zealand's case for an exemption.

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Speaking from Ottawa, Parker said he had presented a "strong case" for New Zealand receiving an exemption but was not able to reach an agreement.

But Parker said this was not a surprise, given trade officials advised him before his trip it would be unlikely the issue would be solved through face-to-face meetings.

"[This is] because no one else has been able to – be it [Japanese] Prime Minister Shinzō Abe, the Europeans or Mexico."

He said he was listened to respectfully by US officials and reached an agreement that New Zealand trade officials would continue to work together on the issue.

National's Trade Spokesman Todd McClay had been applying pressure on Parker to secure the exemptions.

He said the tariffs make it much harder for New Zealand steel and aluminium exporters to get their product into the US.

"The United States is New Zealand's third largest trading partner, however, the threat of escalating tariffs remain an issue of concern for our exporters."

He said Parker "must come back from his trip with a guarantee that New Zealand steel and aluminium exports would be exempt from these US tariffs."

Meanwhile, Parker said the "more pressing issue" for him in the US was protecting the WTO which he said was "under threat."

US President Donald Trump had threatened to pull out of the organisation as he claimed it did not represent US interests.

Parker was advocating for solutions to this issue.

Asked how his conversations went, Parker said: "we live in uncertain times."

"It's not clear what the solution is. We had polite conversations, but we did not resolve the issues, not that we expected to."

Parker said he was pleased to see New Zealand officially ratified the CPTPP earlier today.

"New Zealand's ratification means that from day one our businesses will be able to take advantage of improved trading conditions and lower tariffs."