Leavitt at a congressional panel told the senators that it's just a matter of time that wild birds and possibly poultry flocks would contract the disease.
Leavitt said that a public health emergency would be created if the disease mutated so that it could be easily transferred from human to human.
The H5N1 disease so far has killed 94 people in seven countries.
But the Democrats on the Senate Budget Committee criticized the Bush administration's preparedness, saying that not enough federal funds were being allocated for vaccine production, stockpiling other medical supplies, disease detection and community readiness.
North Dakota Sen. Kent Conrad the senior Democrat on the committee said that the additional $2.3 billion was not enough to tackle the avian flu.
U.S. poultry flocks have suffered from isolated cases of highly contagious avian flu but not from the virulent H5N1 strain that has killed or led to the culling of about 200 million birds, mostly in Asia, since late 2003.
Recently it was found that the avian flu may have jumped species and killed a cat.
It is also seen that human fatalities have largely been limited to people who have had close contact with sick birds.
Leavitt told the committee that by the end of this year, the United States will have about 20 million doses of anti-viral drugs, mostly Tamiflu, stockpiled. He said that the vaccine development is three to five years away.
When the Senate Budget Committee Chairman Judd Gregg, a New Hampshire Republican asked about basic medical equipment Leavitt put the responsibility of local preparedness mostly with local officials.