The Mexican president says he is concerned by the prospect of the US militarising its southern border.
Vicente Fox telephoned the US president to express his fears on Sunday.
According to Fox's office, George Bush told him he was considering sending the National Guard to the border but said this did not constitute a militarisation.
Bush is expected to make a speech on Monday night on how the US should deal with border security and illegal immigrants already in the US.
Fox has lobbied for the US government to legalise some of the 11 million undocumented migrants living in the US, about half of whom are Mexicans.
Most undocumented Mexican migrants enter the US illegally by sneaking over the nearly 3,200km-long common border.
The 12,000-strong Border Patrol force has complained it needs more staff to stop people heading north, particularly in the Sonora desert where thousands of migrants walk in over mountainous and desolate terrain.
Senior US administration officials have told The Associated Press that Bush is considering using federal funds to pay for deploying National Guard troops along the border.
One defence official said military leaders believed the required number of troops could range from 3,500 to 10,000. Another administration official said the 10,000 figure was too high.
The officials insisted on anonymity since no decision has been announced.
Illegal immigrants enter the US
by sneaking through the border
Bush is considering the National Guard plan as he seeks support from conservatives in Congress for his immigration bill.
The president wants to allow foreigners to get temporary work permits to enter and work in the US, but many conservatives want a tougher approach on illegal immigrants.
According to Fox's office, the two presidents agreed that a comprehensive immigration reform is needed in the US.
"In the conversation, President Bush reiterated his conviction that the migration issue can only be resolved with an integral and comprehensive reform," the office release said.