Last week, Mark Astarita, fundraising director at the British Red Cross, said that, as always, the charity would only send in aid if asked.
He said: "The US is the richest country on earth, they can do it themselves, although there may be a need for specialist staff."
The British Red Cross decided to put a link on its homepage to encourage people to donate in dollars to its US counterpart.
On Friday, however, it was asked for more help. On Sunday, it launched an appeal through the national press. In total, the charity is spending £25,000 on the adverts.
Two logisticians from the British Red Cross will travel to the US with more than 80 other disaster experts from 10 Red Cross organisations.
Sky News also offered its help on Friday, by opening its red button interactive option so that people could donate to the British Red Cross.
Astarita said: "The first people to give to the London Bombings Appeal came from the US. We can help return that generosity."
Other UK charities contributing to the effort include the Salvation Army, which sports a donation link on its website, and World Vision, which is sending emergency supplies.