An internal Border Patrol memo obtained by ABC News indicates that the Department of Homeland Security could request up to 10,000 U.S. military troops to help with efforts along the United States-Mexico border.

Defense officials, however, said this week that they have not heard of this figure, but did say they expected that there could be additional requests for troops.

“This is just the start. This is an initial step, and we are anticipating many further missions,” a senior Defense official told reporters.

The White House yesterday announced that 1,500 troops would be sent to help with operations at the southern border. Those roles, according to sources, would be to help with processing and surveillance.

The Customs and Border Protection planning memo also says the agency could request military infrastructure and technology.

CBP “may” utilize military bases as holding facilities for those who are arrested by Customs and Border Protection. Defense officials said that DOD had not received any requests for that kind of assistance but would evaluate such requests.

The Navy may also help with enforcing and carrying out coastal border operations, according to the document.

Two U.S. officials told ABC News on Thursday afternoon that the first of 500 Marines bound for the border would be shipping out in the coming hours and Army units would be in transit later in the day. A good number of the Army troops will be military police, but they will not be carrying out law enforcement duties, according to other officials.

In addition, four military cargo planes are being positioned for use in deportation flights — a C-17 and C-130 in San Diego and another C-17 and C-130 at Fort Bliss, Texas. So far only the C-17 to Ft. Deportation flights cannot begin until the State Department arranges details, which could take some time.

The U.S. Northern Command said in a news release Thursday that it is “aggressively” bolstering security at the border. About 1,500 soldiers and Marines are “immediately” deploying to the region to augment the approximately nearly 2,500 service members already there supporting CBP’s mission at the border.

“In a matter of days, we will have nearly doubled the number of forces along the border, effectively implementing the President’s intent while planning and posturing for expanded efforts to protect the sovereignty and territorial integrity of the United States,” said Gen. Gregory M. Guillot, commander of the Northern Command.

The exact number of personnel will fluctuate as units rotate personnel and as additional forces are tasked to deploy once planning efforts are finalized, Northern Command said. These forces will support enhanced detection and monitoring efforts and repair and emplace physical barriers, the release said.

-ABC News’ Matt Seyler contributed to this report.