Fiona Scott Morton had been nominated to take the role of chief competition economist, a prominent role in the team of Margrethe Vestager, EU competition chief.
Russian President Vladimir Putin warned Moscow has a "sufficient reserve" of cluster munitions and reserves the right to use the weapons in tit-for-tat actions.
The extension will give the watchdog more time to review proposals by the two parties to resolve its concerns after it paused a campaign to block the deal.
The 11-member Comprehensive and Progressive Agreement for Trans-Pacific Partnership includes Canada, Mexico, Japan, Australia, Vietnam, Singapore and Malaysia, among others.
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