Each day the news of Syria gets worse. As a human rights activist told Oxfam, "the situation is hell. In areas under attack, people do not have enough food." Earlier this week, the Syrian Red Cross finally entered Baba Amr with Baroness Amos, but members of the International Committee of the Red Cross, who have been attempting to visit Baba Amr since Friday, remained barred from the neighbourhood.

Like everyone else, I feel horror and frustration at the world's seeming inability to stop the killing and help those in need. Oxfam was founded to support the poor and vulnerable in such crises, and in Turkey, Lebanon and Jordan, Oxfam humanitarian workers urgently prepare for the escalating crisis they expect - for what Oxfam may do as the humanitarian fallout worsens, and more refugees flee across borders.

I watch the news of the UN humanitarian chief Valerie Amos' visit to Baba Amr with hope mixed with dread. Last week she was refused entry to Syria, now she is in the worst affected zone. Will this visit break the impasse and allow aid workers to access the worst-hit areas? Civilians are bearing the brunt of the violence - and living conditions are worsening. Getting aid flowing day after day is critical.

But Oxfam passionately believes the answer to humanitarian crises is more than providing aid and protecting civilians (however vital). As well as preparing for humanitarian fall out, we believe we should be challenging those with the power to resolve this deteriorating crisis, principally the Government of Syria and its supporters (or at least defenders) in the UN and elsewhere.

Last Saturday, UN Secretary General Ban Ki-moon called for the world to speak with one voice. He said that the world's inaction "seems to have encouraged the Syrian authorities in their brutal suppression of its citizens" and called on world leaders to "find unity in pressing the Syrian authorities and all parties to stop the violence" and "insist with one voice that the civilian authorities give access to international aid workers."

We believe it is vital that we back his call. The abuses ordinary men, women and children have faced are unacceptable and we should not be silent in the face of a dire and worsening situation. This is why we are asking everyone to call on the UN Security Council to condemn the violence with one voice, push for unimpeded access for humanitarian aid and to ensure that arms that could fuel further human rights abuses are not supplied.

We have long campaigned for an Arms Trade Treaty, and the flow of arms to Syria underlies its necessity. Both the Syrian government and the Syrian rebels have received arms from outside the country. Increased weapons and ammunition have exacerbated the conflict, and increased civilian suffering.

There is no UN embargo on arms to Syria. The EU has an arms embargo on Syria which dates to May 2011, but binds only its member states. Since Syria purchases no arms, ammunition or equipment from the EU this embargo is of limited effect. Clearly action is needed to prevent more arms getting in.

Nothing is more important than stopping the killing on all sides. Unlike many crises we face, with a tangled mix of political and 'natural' causes, Syria is a political crisis caused by a government refusing its people's right to be heard. Easy to solve? Of course not. But its direct political cause does mean it is amenable to international political pressure - if the world has the will to exert it.

So far, that pressure has been fatally undermined. But we need to keep the faith that there is a level of international outrage that can persuade the Syrian government's international defenders that they must act to stop the killing now. While we plan what we might do on the ground, NGOs like Oxfam must be part of that global tide of outrage, joining the call for an end to the killings, the arms supplies that fuel them, and immediate access for humanitarian aid.

In concert with others, we hope that our voices and those of our supporters can make a difference. We also know that civilians struggling in crises welcome a solidarity that reinforces their sense that they are not alone. The situation in Syria is at breaking point, we need to speak out before it gets worse still.

To add your voice to Oxfam's please visit our webste