Speaking after talks with Mr Portillo in Bucharest, Romania's Defence Minister, Gheorghe Tinca, said the idea of expanding Nato in waves was flawed and that his country feared the first wave of enlargement would turn out to be the last.
"If the idea of a wave is accepted, then I am convinced there will be just one," Mr Tinca told the daily Adevarul. Mr Portillo, on the first leg of a regional tour, said Nato would not slam the doors on further members once the first new entrants were safely on board.
Nato member states are expected to announce early next year that Poland, Hungary, the Czech Republic, and possibly Slovenia, will be invited to join the alliance, perhaps by 1999.
Despite reforms of its armed forces, Western officials say Romania has not progressed far enough in the overall reform process to qualify for early membership and that, together with Slovakia, Bulgaria, Albania and the Baltic states, it will have to wait for a second, or a third, wave of expansion.
Bucharest fears exclusion from the alliance could pull the country into a "grey zone", in which it could once again fall under the sway of Moscow.
Romania is particularly annoyed at the prospect of Hungary joining Nato first, arguing that once it is in, Budapest may exercise a veto over Romania's membership.
In an attempt to stymie Hungary's application, the Romanian authorities have warned that any "discrimination" in Hungary's favour would lead to a climate of "competition, mistrust and instability" in the region and could even spark an arms race.
Hungary denies that it would try to block Romania's entry into Nato. It argues that as Romania has a large Hungarian minority, it would be in Hungary's interests for Bucharest to join.
tBonn - News reports say Germany and other Nato nations are planning for the alliance to extend the presence of peace- keeping forces in Bosnia past the end of the year, to prevent the renewal of conflict in the region, AP reports.
The plans do not discount placing German fighting troops in the region, the Welt am Sonntag newspaper said.