PARIS  - Turkey is considering invoking the NATO alliance’s mutual defence treaty over “outrageous” Syrian shelling along its border, US Secretary of State Hillary Clinton said Thursday.“Turkey is considering formally invoking Article Four of the North Atlantic Treaty,” Clinton said, at a meeting in Paris of senior envoys from countries seeking to pressure Bashar al-Assad’s Syrian regime.Article Four states that members “will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened”.It is less strong than Article Five of the NATO treaty, under which members invoke the right to “individual or collective self-defence”, but would be a step towards a collective response to alleged Syrian aggression.Hillary Clinton said world powers should push for tough United Nations sanctions against Syria in order to force Bashar al-Assad’s regime to comply with a UN peace plan.“We need to start moving very vigorously in the Security Council for a Chapter Seven sanctions resolution,” Clinton told international envoys at talks in Paris.A Chapter Seven resolution, which can be imposed by the Security Council if member states think peace is threatened by an act of aggression, authorises foreign powers to take measures — including military options.Clinton said Chapter Seven measures could involve “travel, financial sanctions, an arms embargo, and the pressure that will give us on the regime to push for compliance with Kofi Annan’s six-point plan”.But Clinton admitted that Russia — which has a veto on the Security Council — would probably not allow such a motion to pass, and said in the meantime states would have to seek further diplomatic and economic sanctions.One measure, she said, would be for NATO to invoke its mutual defence treaty in response to “outrageous” shelling by Syria on its northern border with Turkey, a member of the NATO alliance.“Now, I’m well aware that at this point such an effort is still likely to be vetoed, but we need to look for a way to keep pressing forward,” she said, adding she had earlier met her Russian counterpart Sergei Lavrov in Brussels.“He was, as usual, very intent upon laying responsibility on all sides, and in particular on the opposition, but he also has recognised that we are not in a static situation but a deteriorating one,” she said.“Next, we have to keep Assad off balance by leaving options on the table.“And Turkey already has discussed with Nato ... over the last two days the burden of Syrian refugees on Turkey, the outrageous shelling across the border from Syria into Turkey a week ago,” she said.