WASHINGTON — The United States is significantly stepping up its support for the Syrian opposition, senior administration officials said on Wednesday, helping to train rebels at a base in the region and for the first time offering armed groups nonlethal assistance and equipment that could help their military campaign.

The training mission, already under way, represents the deepest American involvement yet in the Syrian conflict, though the size and scope of the mission is not clear, nor is its host country. The offer of nonlethal assistance is expected to come from Secretary of State John Kerry at a meeting on Thursday in Rome with opposition leaders. Mr. Kerry is also expected to raise the prospect of direct financial aid, though officials cautioned that the White House still had to sign off on all the elements.

Before arriving in Rome on Wednesday, Mr. Kerry declared in Paris that the Syrian opposition needed additional assistance and indicated that the United States and its partners planned to provide some.

Under a broad definition of “nonlethal,” assistance to the opposition could include items like vehicles, communications equipment and night vision gear. The Obama administration has said it will not — at least for now — provide arms to the opposition.