He told reporters that Poland would not endorse any "proposals or decisions prepared in Moscow" as part of US-brokered talks with Russia, Polish state news agency PAP reported.
Tusk was speaking on the sidelines of a summit of the European Political Community in Dublin, where he met with Ukrainian President Volodymyr Zelensky.
Their talks focused on negotiations between Ukraine and Russia held in Abu Dhabi this week under US mediation.
"Negotiations are taking place, although I think that is too strong a word," Tusk said.
He added that Ukraine needed unified European support, telling reporters that "the goals of US diplomacy may differ somewhat from what Europe expects."
Tusk said he saw no divisions among EU countries over support for Ukraine in peace talks.
Any settlement, he said, must preserve Ukraine's sovereignty and leave the security architecture of eastern Europe unchanged.
"Europe will under no circumstances allow new security rules for this part of the world to be decided over our heads," he said.
"We will not build a new security architecture simply because the Russian president or, for example, someone in the United States wants it," he added.
Tusk also said he and Zelensky had discussed Ukraine’s bid to join the EU.
Zelensky had stressed the importance of ensuring that the accession process did not undermine Polish interests, including in agriculture, Tusk said.
Asked who should represent Europe in peace negotiations, Tusk said he would favour strengthening a common EU position rather than creating new formats.
"We have European institutions. We have the European Commission, the president of the European Council and the high representative for foreign affairs," he said.
Tusk added that he did not want Poland to "join a contest" over who should represent Europe.
He said he remained in regular contact with European partners and had "no shortage of opportunities" to take part in discussions.
The Polish prime minister said, however, that Poland was "not welcome" at the negotiating table organised by US diplomats under President Donald Trump, the PAP news agency reported.
He warned that accepting Moscow's terms would pose a direct threat to Poland's security.
"I am in constant contact with everyone involved in this process, but I would not want Poland to endorse proposals or decisions prepared in Moscow," he said.
EU leaders are due to discuss the peace process at an informal European Council meeting in Dublin on Friday, including who should represent the bloc in negotiations.
The Abu Dhabi talks brought together Ukrainian and Russian officials under US mediation.
No European delegation took part, although European officials held separate consultations with Ukraine and the United States.
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Source: IAR/PAP