He told reporters on Monday that "no diplomatic optimism" should weaken Europe’s focus on security.
Commenting on a recent joint appearance by US Secretary of State Marco Rubio and top Ukrainian presidential aide Andriy Yermak, Kosiniak-Kamysz called it a "positive signal," but warned that Ukraine must remain "a subject, not an object" of any peace talks.
He said even encouraging messages from ongoing negotiations must not slow Poland's push to bolster its defences.
"We need to keep arming ourselves; we need to improve our security," he said, adding that he would "repeat like a mantra" the need to shield defence spending from cuts.
"We must accelerate modernisation even further, in an even shorter time," he said.
Rubio and Yermak said after talks in Geneva on Sunday that they had held their "most productive meeting" so far on ending the war, and that both sides were working on revisions to Washington's original 28-point peace proposal.
They reported "significant progress" and said they were moving toward "a just and lasting peace."
The Axios news service reported last week that the US plan, in its original form, would require Ukraine to forgo NATO membership, cap its armed forces at 600,000 troops and cede some territory to Russia.
The Reuters news agency reported on Friday that Washington had threatened to limit intelligence sharing and weapons deliveries to push Kyiv towards accepting the deal.
US President Donald Trump has given Ukraine until Thursday to respond, though he later said the proposal was not his final offer.
European leaders, meeting in Luanda on Monday and in Geneva the day before, have put forward counterproposals, including financial compensation from Russia to Ukraine and the freezing of Russian assets.
(ał/gs)
Source: PAP, Reuters, state.gov