KIEV, Jan. 26 - Ukraine may declare invalid the recent Russian-Ukrainian natural gas deal if its investigators discover that it has been signed under pressure, a top energy security aide to President Viktor Yushchenko said Monday.

Bohdan Sokolovskiy released comments days after Oleh Dubyna, the head of the state gas company Naftogaz Ukrayiny that had signed the deal, was hospitalized in serious condition after suffering a heart problem.