The 20 women serving in the U.S. Senate are on the verge of asking President Obama to take further steps to condemn the kidnapping of hundreds of Nigerian girls, according to one of the senators leading the effort.

The kidnappings that occurred last month have drawn global condemnation, especially through social media, where #BringBackOurGirls has become a trending topic of conversation on Twitter and other platforms. The senators' efforts come as the leader of Boko Haram, the Nigerian Islamic extremist group, is threatening to sell the nearly 300 teenage schoolgirls abducted from a school in the remote northeast section of the country. Abubakar Shekau has also claimed responsibility for the April 15 mass abduction and is warning that his group may attempt to kidnap more girls.

On Tuesday morning, Sens. Susan Collins (R-Maine) and Barbara Mikulski (D-Md.) were seen gathering signatures on the Senate floor from the 18 other women who serve in the Senate. Repeatedly, Collins and Mikulski approached their colleagues, handed them a pen and implored them to sign. Each of the women senators did and several were seen agreeing with Collins and Mikulski that the situation is "horrible."

Collins said later that the situation is "so horrendous and the comments made by the head of this group that girls should be married between ages 9 and 12 and should be denied any kind of education call out for a vigorous response from all around the world -- men and women alike. But I think having the 20 women senators lead the way is the beginning of sending very powerful signal. It's not sufficient, but it's a first step of the actions we want to take."

Collins said the letter would request that President Obama ask the United Nations to classify Boko Haram as a terrorist organization and take steps to impose sanctions against the group. The State Department already classifies the organization as a terrorist group.

Collins said she and her colleagues are also mulling "additional steps" that could be taken. She said every one of her colleagues is "utterly appalled that this could happen in this day and age and that it appears so little is being done."